Acoustic wave manipulation by means of a time delay array

ABSTRACT

A device ( 20 ) for manipulating an incident acoustic wave to generate an acoustic output is described wherein the device comprises a plurality of unit cells arranged into an array, at least some of said unit cells being configured to introduce time delays to an incident acoustic wave at the respective positions of the unit cells within the array of unit cells, such that said plurality of unit cells define an array of time delays to thereby define a spatial delay distribution for manipulating an incident acoustic wave to generate an acoustic output ( 30 ). The array of time delays may be re-configured to vary the spatial delay distribution of the device in order to generate different acoustic outputs. Also described are methods for designing or configuring such devices.

The present invention relates generally to devices for manipulatingacoustic waves.

BACKGROUND

The ability to manipulate acoustic waves may be important in variousfields including, but not limited to, noise control, power charging,loudspeaker design, position/motion sensing, ultrasound imaging andtherapy, non-destructive testing of engineering structures, hapticcontrol utilising focussed acoustic waves (i.e. haptic user interfaces)and acoustic particle manipulation e.g. acoustic levitation. Theseapplications generally require more precise control of acoustic waves.

Current approaches for manipulating acoustic waves rely on fixed lenses,performing only a single function, or a phased array of transducerswherein the amplitudes and phases of the individual transducers withinthe array are independently controlled. For instance, the amplitudes andphases may be controlled either by controlling the relative positions ofthe transducers within the array, within a fixed geometry, or byintroducing a phase delay by triggering the individual transducers atdifferent points in time. The latter approach is generally the preferredimplementation for consumer electronic devices. However, phased arraysare typically bulky and expensive to control or manufacture, and are noteasily scalable as the cost and complexity of the associated electronicsgenerally scales with the number of channels.

Despite these limitations, phased arrays are in widespread use.

For example, sparse arrays of phased transducers are used to treat avariety of tumours or functional brain disorders in High FrequencyFocussed Ultrasound techniques by inducing a localised heating effect.Such arrays are also used in industrial applications where focussing andsteering of acoustic or ultrasonic waves may be used to find smallcracks in metallic components having complex and highly anisotropicgeometries.

It is desired to provide improved techniques for manipulating acousticwaves.

SUMMARY

According to a first aspect of the present invention, there is provideda device for manipulating an incident acoustic wave to generate anacoustic output comprising: a plurality of unit cells arranged into anarray, at least some of the unit cells being configured to introducetime delays to an incident acoustic wave at the respective positions ofthe unit cells within the array of unit cells, such that the pluralityof unit cells define an array of time delays to thereby define a spatialdelay distribution for manipulating an incident acoustic wave togenerate an acoustic output, wherein the array of time delays defined bythe plurality of unit cells is re-configurable to vary the spatial delaydistribution in order to generate different acoustic outputs.

The present invention (in any of its aspects and embodiments) relatesgenerally to a novel approach for manipulating acoustic waves using adevice comprising a plurality of unit cells each capable of encoding aparticular time delay (or set of time delays).

It will be appreciated that the unit cells that are configured tointroduce time delays have the effect of slowing down acoustic wavespassing through the device. Each unit cell may be configured tointroduce a particular time delay to an acoustic wave passing throughthe unit cell.

For instance, the physical structure of the unit cells may be designedso as to cause the acoustic waves to travel an extended effective pathlength, L_(eff), so that it takes longer for the acoustic waves totransit the unit cell than it would if the acoustic waves travelleddirectly from one side of the unit cell to the other. Thus, in preferredembodiments, the respective time delay for each of the unit cells isdetermined by the path length through the unit cell. Each of the unitcells may therefore have an associated path length. By changing the pathlength at a particular location, or e.g. using unit cells with differentpath lengths, the spatial delay distribution of the device can thus bechanged.

Additionally, or alternatively, in some embodiments the unit cells maybe configured so that the speed of sound, c, within the unit cell ischanged (i.e. reduced) relative to the speed of sound in the ambientmedium. In general, the time delay introduced by the unit cells may beof the order Δt˜L_(eff)/c. Depending on the design of the unit cells,the time delay may depend on the frequency of the incident acoustic waveor may be essentially frequency independent.

For instance, preferably, the unit cells are filled in use with the samefluid (e.g. air or water) within which they are operating. That is,preferably the unit cells are substantially open to allow thesurrounding fluid to pass into or through the unit cells. However, insome embodiments, it is contemplated that a different fluid may beprovided within the unit cells, e.g. to further modify the properties ofthe incident acoustic wave.

U.S. Pat. No. 6,554,826 (TxSonics-LTD) describes an alternative approachwherein an acoustic lens is provided comprising an internal mediumhaving voltage dependent acoustic properties which can be controlled byapplying suitable voltages to various electrode surfaces. However, atleast compared to embodiments of the present invention, e.g. where thetime delays for the unit cells are determined (at least in part) by therespective acoustic path lengths within the unit cells, this approachmay be relatively inefficient (for instance, transmission rates may belimited) and have a more limited range of operation (for instance, theapproach described in U.S. Pat. No. 6,554,826 is only linear in certainoperating ranges).

It will be appreciated that the effect of the time delays is that for anincident acoustic wave at a particular frequency the unit cells willintroduce a phase delay, wherein the phase delay angle is given byΔφ=k.L_(eff), where k is the wavenumber of the incident wave. That is,the phase delays are generally frequency dependent. Thus, it will beunderstood that where reference is made herein to a “time delay”, thismay alternatively be considered as a “phase delay” that depends on thefrequency of the incident acoustic wave and that the time delay andphase delay values may be related to each other depending on theoperating frequency or frequencies.

The different unit cells within the array may be configured so as tointroduce different time delays. An acoustic wave incident on andpassing through the device may thus be subject to various different timedelays at the positions of the unit cells within the array. Inparticular, the unit cells are arranged together in an array such thatthe positions of the unit cells and their associated time (or phase)delays define a spatial delay distribution across the surface of thedevice. It is this spatial delay distribution that determines how anacoustic wave incident on the array of unit cells will interact with andbe manipulated by the device.

The surface of the device is thus effectively spatially quantisedaccording to the positions and dimensions of the unit cells. Thedimensions of the unit cells effectively define the resolution at whichthe surface is quantised in the spatial domain. In the preferredembodiments, as explained further below, the unit cells may each encodeone or more discrete time or phase delay values, so that the delaydistribution is also quantised in the time or phase domain. It will beappreciated that the time or phase delay for a particular unit cell maybe zero, and that the array(s) may also contain spaces or empty cells.

By controlling the positions and/or time delays of the (individual) unitcells within the array, the device may be selectively configured toperform various manipulations of the incident acoustic waves, and bychanging the positions and/or time delays of the unit cells within thearray, the device may be re-configured to perform a differentmanipulation.

It will be appreciated that the devices according to embodiments of thepresent invention may therefore provide a flexible and/or low-costapproach for manipulating acoustic waves to generate essentiallyarbitrary acoustic outputs. This re-configurability may be facilitatedby a modular structure of the device, which is formed by an arrangementof unit cells. For instance, in embodiments, the positions of thedifferent unit cells within the array and/or the time delays associatedwith the unit cells may be varied in order to control the acousticoutput of the device. In some embodiments, the techniques describedherein may facilitate simplifying the design and control of acousticdevices and systems.

For instance, it will be appreciated that the devices according toembodiments of the present invention may provide various benefitscompared to current transducer-based approaches. Other approaches formanipulating acoustic waves are described e.g. in U.S. Pat. No.5,546,360 (Deegan) and U.S. Pat. No. 5,477,736 (General ElectricCompany) which both describe providing an acoustic lens including anelectro rheological fluid. However, in both cases, it is not possible toprovide local changes in the spatial delay distribution and the devicesdescribed in these documents may therefore be relatively limited, atleast compared to embodiments of the present invention. A similarapproach is described in US 2013/0112496 (University of North Texas). US2014/0060960 (University of North Texas) describes yet another approachwherein a tunable polymer-based sonic structure periodic structure (i.e.a sonic crystal) is provided.

The unit cells may generally take various suitable forms. For example,each of the plurality of unit cells will typically (and preferably does)comprise a central channel extending from one side of the unit cell tothe other to allow acoustic waves to pass through the unit cell.However, the central channel may be structured, and the interactions ofthe incident acoustic waves with this structure may increase theeffective path length for the acoustic waves travelling through the unitcell, and thereby introduce a time delay. Particularly, the unit cellsmay each comprise a collection of structures with which the incidentacoustic wave is caused to interact, with the size of the structurestypically being smaller than the wavelength of the incident acousticwave. For example, the central channel may have a substantiallylabyrinthine or meandered structure that determines the respective timedelay for the unit cell. In other embodiments, the unit cells maycomprise a multi-slit, helical, coiled or Helmholtz-resonator typestructure. The structure may generally be symmetric about a plane ofsymmetry through the central channel (but need not be).

According to embodiments of the present invention, the unit cells maygenerally comprise of “acoustic metamaterials”. Acoustic metamaterialsare generally characterised by their effective mass density and bulkmodulus. The structure of an acoustic metamaterial may be engineered toperform various manipulations, and may for instance be engineered tohave negative effective parameters leading to interesting effects suchas negative refraction and sub-diffraction focussing. In this context,the metamaterials effectively slow down or speed up the sound waveshence altering the effective speed of sound and/or path length withinthe material. Current studies of acoustic metamaterials are typicallylimited to audible frequencies up to 20 kHz, and are designed toillustrate a specific principle, or to fit a specific purpose e.g. alens with a fixed focus. That is, acoustic metamaterials are typicallycurrently only used to create relatively limited, static structures. Bycontrast, the present invention presents flexible solutions formanipulating potentially arbitrary acoustic waves.

According to a first aspect of the present invention, the unit cellsallow the device to be re-configured in order to vary the distributionof time delays across the device.

Two main embodiments for re-configuring the array of time delays willnow be described.

In the first main embodiment, the plurality of unit cells comprises aplurality of pre-configured unit cells, each pre-configured unit cellbeing configured to introduce a fixed time or phase delay to theincident acoustic wave.

According to the first main embodiment, the array of time delays maythus be re-configured by changing the type and/or position of at leastsome of the pre-configured unit cells within the array of unit cells.

That is, at least some of the plurality of unit cells within the arraymay be pre-configured unit cells encoding a fixed time or phase delay.For instance, the plurality of unit cells may comprise a plurality ofdifferent types of unit cell, with the different types of unit cellencoding a different time or phase delay (e.g. having a differentassociated path length). Each type of unit cell may be unique. Thedevice may thus be readily re-configured by physically re-arranging theunit cells within the array into a different arrangement of delayvalues. For instance, the unit cells may be manually re-arranged by auser. However, it is also contemplated that the unit cells (or theirarrangement) may be mechanically re-arranged by a machine or accordingto a control signal. In any case, by selecting the appropriatearrangement of the pre-configured unit cells, the spatial delaydistribution of the device may easily be controlled and re-configured asdesired. According to this embodiment, the pre-configured unit cellsessentially act as building blocks for the device according to thepresent invention.

The unit cells may generally be self-supporting. However, inembodiments, individual unit cells may be configured to be releasablymounted within a support structure. Thus, the device may furthercomprise a frame or mounting structure, wherein the plurality of unitcells may be releasably mounted on or within the frame or mountingstructure. For example, the pre-configured unit cells may be insertedinto respective positions within a grid structure in order to define thearray. In other embodiments the unit cells may be configured for mutualinterconnection with each other. Particularly, the unit cells may bereleasably interconnectable with one another. For instance, the unitcells may be configured such that different unit cells may be clippedtogether, or otherwise interconnected, in order to define the array. Inthis case a separate frame or mounting structure may not be required.

In embodiments, a number of unit cells may be joined together into a‘block’.

Thus, the array of unit cells may comprise a plurality of blocks of unitcells, each block comprising a fixed arrangement of unit cells. Inparticular, a (or each) block of unit cells may comprise a fixedarrangement of pre-configured unit cells arranged so as to provide apre-determined manipulation of an incident acoustic wave. A (or each)block of unit cells may, for example, comprise an array of unit cells,such as a square or rectangular array of 3×3 or 9×9 unit cells. However,it will be appreciated that a (or each) block of unit cells maygenerally comprise any suitable fixed arrangement of unit cells. Forinstance, a (or each) block of unit cells may comprise athree-dimensional array of unit cells, such as a cubic array of 3×3×3unit cells.

Thus, instead of choosing the positions and/or delay values of theindividual unit cells within the array, in embodiments, a set of blocksmay be used to construct the array. For example, each block of unitcells may comprise a fixed and pre-determined arrangement of unit cellssuch that each block may act to provide a certain pre-definedtransformation or manipulation of an incident acoustic wave. Thus, ablock may be designated e.g. as a ‘steering’ block, as a ‘focussing’block, or as a ‘half-wave’ block. It will be appreciated that the use ofsuch blocks may facilitate mechanical assembly of the device.

As mentioned above, according to the first main embodiment the unitcells (or blocks of unit cells) may essentially act as a set of buildingblocks from which the array can be created. A suitable kit of unit cellsor blocks may be sold together to a user depending on their requiredspecification.

In embodiments, the first aspect may also extend to a kit of parts forforming a device substantially as described herein according to thefirst main embodiment of the first aspect, the kit comprising aplurality of different types of unit cell or blocks of unit cells, and aframe or mounting structure for mounting the plurality of differenttypes of unit cell or blocks of unit cells in an array.

Accordingly, a kit of parts for forming a device substantially asdescribed herein according to the first main embodiment of the firstaspect may be provided comprising a plurality of different types of unitcell or blocks of unit cells, wherein the plurality of different typesof unit cell or blocks of unit cells may be mutually interconnected todefine an array of unit cells.

The pre-configured unit cells may each be configured to encode aspecific time delay and/or a specific phase delay.

For instance, in embodiments, the unit cells are each designed to encodea specific phase delay at a particular operating frequency of interest.That is, the structure of the unit cells may be designed to introduce adesired phase delay at a selected frequency. For the selected frequency,a typical set of pre-configured unit cells may be arranged to span thephase delay range 0 to 2π in discrete intervals. For instance, thepre-configured unit cells may be configured to span the phase delayrange 0 to 2π in uniform intervals of (e.g.) π/8. Thus, in that case, 16unique pre-configured unit cells may be available for forming thedevice. It has been found that using 16 unique phase delays allows thereproduction of essentially any desired acoustic wave with a precisionof about 0.1 dB. However, in general, there may be fewer or greaterunique types of unit cells, as desired. For instance, in embodiments, 8different types of unit cells may be provided and this may be sufficientfor the same precision of about 0.1 dB. In embodiments, the unit cellsmay span the phase delay range 0 to 2π in non-uniform intervals. Thismay reduce the number of unit cells needed to reproduce a desired field,particularly where the unit cells may be stacked i.e. added together. Akit, as may be provided to a user, may comprise a variety of types ofunit cells. For example, a kit may comprise at least one of eachdifferent type of unit cell within the set, or the unit cells within thekit may be determined or substantially optimised (using the techniquespresented herein) to include only the, or the minimum or optimum numberof, unit cells required for a particular application, e.g. based on adesired acoustic output and/or accuracy.

It will be appreciated that the unit cells may alternatively, oradditionally, be designed to encode a specific time delay. Particularly,the unit cells may be designed to introduce a fixed time delay that isessentially independent of the operating frequency. The phase delays inthis case will depend on the operating frequency or frequencies. Thearrangement of unit cells within the array may thus be selected based onthe time delays and a specified or selected frequency in order to givethe desired phase delays.

In a second main embodiment of the first aspect, the plurality of unitcells comprise a plurality of re-configurable unit cells that may eachbe selectively (controllably) re-configured to cause the unit cell tointroduce different time (or phase) delays. Optionally, each of theplurality of re-configurable unit cells may be selectively and/orindependently re-configured. However, in other embodiments, a block orgroup of re-configurable unit cells may be re-configured together. Insome embodiments, the unit cells may be capable of being re-configuredsubstantially continuously between a range of phase delay values.However, preferably, a (and each) re-configurable unit cell may beselectively re-configured between a set of two or more discrete phasedelay values, such as two, four, eight or sixteen discrete phase delayvalues. In some preferred embodiments, the or each re-configurable unitcell may be selectively re-configured between only two discrete timedelay values.

For instance, according to the second main embodiment, the array maycomprise a fixed arrangement of unit cells wherein each unit cell withinthe array may be re-configured to cause a different time delay in orderto change the delay distribution of the device and generate differentacoustic output fields.

It will be appreciated that the approach of the second main embodimentlends itself to electronic control. For instance, using computercontrol, the delay distribution of the device may be re-configuredpractically in real-time to change the form of the acoustic outputfield. It will also be appreciated that the electronic controlrequirements for re-configuring the device may be relatively simple e.g.compared to the electronic control requirements for a conventionalphased array transducer particularly because the electronic control ofthe device may be separated and independent from the power requirementsof any acoustic source used to generate the incident acoustic waves.

The device may thus further comprise one or more electrodes or electrodelayers for providing control signals for re-configuring there-configurable unit cells. The re-configurable unit cells may beconnected to the one or more electrodes or electrode layers and thecontrol signals may be provided via the one or more electrodes orelectrode layers to the re-configurable unit cells. The device mayfurther comprise (or may be connected in use to) an electroniccontroller and/or processor for re-configuring the arrangement. Thecontroller and/or processor may generate electronic control signals forre-configuring the unit cells.

It will be appreciated that a (or each) re-configurable unit cell maytake various suitable forms. However, typically, each of there-configurable unit cells may comprise one or more moveable elementsmoveable between a plurality of positions in order to vary the timedelay introduced by the re-configurable unit cell. For instance, theunit cells may each comprise one or more bars or flaps that can beselectively moved into a central channel of the unit cell in order tointroduce a meandered structure and to thereby introduce an additionaltime delay, as described above.

It is contemplated that the device may comprise a mixture ofpre-configured and re-configurable unit cells. The device may thuscomprise at least one (or a plurality of) pre-configured unit cellsaccording to the first main embodiment and at least one (or a pluralityof) re-configurable unit cells according to the second main embodiment.It is also contemplated that at least some of the re-configurable unitcells may be removable, or that some of the unit cells may be fixed inposition and introduce a fixed time delay.

In general, the unit cells described herein may be formed according toany suitable and desired manufacturing techniques. For instance, inembodiments, it is contemplated that the unit cells may each be formedas individual structures, e.g. using an additive (“bottom-up”)manufacturing technique such as 3D printing, and then assembledon-demand into an array structure as desired. For example, each unitcell may be fabricated monolithically as a single structure comprisingan acoustic channel suitably designed to encode a desired time delay.

As another example, the unit cells may be fabricated using microfluidictechniques such as etching or stereo-lithography.

However, in some preferred embodiments, the unit cells may be fabricatedas, or from, a stack of (relatively thin) layers. That is, the unitcells may (each) comprise a plurality of layers that are stackedtogether to define the desired structure to encode a particular timedelay. The structure of a or each unit cell (and hence the associatedtime delay) may thus be defined by a plurality of layers in combination.

This technique may be used to fabricate individual pre-configured unitcells encoding a fixed time delay, e.g. for use with the first mainembodiment of the first aspect of the invention.

This layered construction technique also presents a way of constructingre-configurable unit cells of the type described above in relation tothe second main embodiment of the first aspect of the invention. Forexample, because the unit cells are formed from a plurality ofindividual layers, it is possible to move (e.g. slide) one or more ofthe layers of a unit cell relative to the other layers of that unit celland thereby change the structure of the unit cell to adjust the timedelay that is introduced.

This layered construction for the unit cell is also considered to benovel and advantageous in its own right, i.e. regardless of whether ornot the device comprises a plurality of unit cells that can bere-positioned or re-configured to vary the spatial delay distribution ofthe device as per the first aspect of the invention described above.Furthermore, instead of using a plurality of stacked layers to constructindividual unit cells, it would also be possible with suitably designedlayers to construct acoustic surfaces comprising an array (plurality) ofunit cells. For example, each layer may comprise a portion (or “slice”)of each of the unit cells within the array. Thus, when the layers arestacked together to form an acoustic surface, the array of unit cells(and the structures of each of the unit cells within the array) may bedefined by the plurality of layers in combination. Various othersuitable arrangements are also possible.

Accordingly, from a second aspect of the present invention, there isprovided a device for manipulating an incident acoustic wave to generatean acoustic output comprising: a plurality of unit cells arranged intoan array, at least some of said unit cells being configured to introducetime delays to an incident acoustic wave at the respective positions ofthe unit cells within the array of unit cells, such that said pluralityof unit cells define an array of time delays to thereby define a spatialdelay distribution for manipulating an incident acoustic wave togenerate an acoustic output, wherein the structures of the unit cellswithin the array are defined by a plurality of layers in combination.

That is, the array of unit cells generally comprises a plurality oflayers that are stacked together such that structures of the unit cellswithin the array are defined by the plurality of layers in combination.

By utilising a layered construction for the unit cells (whetherindividually or for the array of unit cells as a whole) it is believedthat various advantages may be provided, at least in some embodiments,in terms of both manufacturability and acoustic performance.

For instance, a layered construction may typically be both faster andcheaper compared to traditional additive fabrication processes such as3D printing. For example, each of the layers may be fabricated usingexisting “top-down” MEMS fabrication techniques where bulkmicro-machining processes (e.g. laser cutting, chemical wet etching,photolithography, etc.) can be used to pattern each of the layers asdesired. For instance, such processes may be used to selectively removematerial from the layer in order to define a pattern of one or moreopenings in each layer. Each layer may therefore have an essentiallytwo-dimensional (or ‘binary’) structure defined by these openings (andthe lack of openings). By appropriately stacking a plurality of theselayers together, e.g. with the openings suitably aligned or overlappingon adjacent layers, it is possible to construct a three-dimensionalstructure. That is, a three-dimensional unit cell structure may befabricated by stacking together a plurality of essentiallytwo-dimensional (planar) layers. The openings in each of the layers maythus define in combination an open channel extending through the unitcell through which acoustic waves may propagate.

Furthermore, in embodiments, these techniques may be able to providehigher resolution features than would typically be possible e.g. whenusing an additive approach such as 3D printing which would be limited bythe resolution of the printer, and for current technologies would belimited to features on the scale of tens of microns. For instance, andby contrast to traditional additive manufacturing techniques, existingtechniques for fabricating two-dimensional layers (e.g. MEMS ormicrofluidic techniques) may be used to define (sub) micron features oneach of the layers. The structure of a unit cell formed in this way maytherefore include relatively smaller features, e.g. suitable formanipulating shorter wavelengths. Thus, the layered construction,wherein a unit cell is fabricated from a plurality of layers that arethen stacked together to define the structure of the unit cell mayprovide an approach for fabricating sub-wavelength structures that isscalable across a wider range of different operating wavelengths, e.g.even for relatively shorter operating wavelengths.

The layered construction may therefore open up a relatively largeramount of control over the propagation of acoustic waves within the unitcells, in a relatively compact and efficient manner. For instance,because it is relatively easier to incorporate smaller features into thechannel structure, it may be possible to produce a larger range ofdifferent types of unit cells (with different time delays).

It will be understood that the unit cells of a device according to thesecond aspect may generally, and preferably do, comprise any of thefeatures of the unit cells described above in relation to the firstaspect (at least to the extent that they are not mutually exclusive).

For instance, where a unit cell, or an array of unit cells, isfabricated from a stack of layers, a (structured) channel may be formedthat extends through each unit cell and that determines the associatedtime delay for that unit cell, e.g. in the same manner described above.The channel may generally extend from one side of the unit cell to theother (e.g. in the direction of propagation of the incident acousticwaves, e.g. the “z-direction”).

For example, and preferably, each of the layers may comprise one or moreopenings such that when the layers are stacked together the openings inadjacent layers are aligned or overlapped so as to define a channelextending through the unit cell. Thus, by appropriately designing orselecting the size and position of the openings provided on each of thelayers that are to be stacked together to define a unit cell, the shapeof the channel for that unit cell may be controlled as desired. Inembodiments, the channel may be substantially labyrinthine or meanderedto define an extended effective path length for determining the timedelay that is introduced by the unit cell.

The shape of the channel may be defined in essentially two dimensions(with the channel then being extended, or extruded, into the thirddimension to define the unit cell). That is, the channel may be foldedin two dimensions (only) (whereas the shape of the channel isessentially unchanged in the third dimension). Thus, the channel mayhave a substantially labyrinthine or meandered two-dimensionalcross-section (e.g. in one of the planes parallel to the direction ofpropagation of the incident acoustic waves, e.g. the “x-z” or “y-z”plane).

However, it will be appreciated that the use of a layered constructiontechnique also allows for more complex (three dimensional) channelgeometries to be created. For example, the channel may be folded inthree dimensions, e.g. to define a helical, or otherwisethree-dimensional labyrinth or meander structure within the unit cell.The use of such three-dimensional channels may provide variousadvantages both from an acoustic and manufacturability perspective.Particularly, by folding the channel in a third dimension, so that alonger physical path length can be created in a smaller volume, it maybe possible to reduce the number of the layers in the stack needed tointroduce a desired time delay. Thus, the device may be furtherminiaturised, e.g. to yield (ultra) sub-wavelength thickness.

It will be appreciated that the array of unit cells of the device maygenerally be arranged in a plane or surface (which is typically flat ortwo-dimensional, but may in some cases be curved). For instance, thearray of unit cells may typically define a plane or surface that issubstantially normal to the propagation direction of an acoustic inputwave provided to the device.

The plurality of layers that form the unit cell(s) may be stacked eitherparallel or perpendicularly to the plane and/or surface of the array.

For example, where the array extends generally in a “horizontal” (first)plane, the unit cells may be built up by stacking a plurality of layershorizontally (in the first plane), e.g. normal to the propagationdirection of the acoustic input wave. Thus, each of the layers maycomprise one or more openings, with the openings in adjacent layers thenbeing aligned or overlapped to define a channel extending from one sideof the unit cell (e.g. from the bottom layer) to the other side (e.g.the top layer). In this case, it will be appreciated that because thechannel shape is defined by a plurality of layers together, the channelmay be limited (in the “vertical” direction extending through the unitcell) to features having a resolution corresponding to the thickness ofthe layers.

Thus, it is also contemplated that the unit cells may be fabricated bystacking together a plurality of “vertical” layers, i.e. layers thatextend perpendicularly to the (first) plane and/or surface of the array,e.g. parallel to the propagation direction of the acoustic input wave.For example, in this case, each layer may comprise a channel structureextending from one side of the layer to the other (parallel to thepropagation direction of the acoustic input wave), with the layers thenbeing stacked together to extend the channel structure into the thirddimension to define a three-dimensional structure. This may allow moreflexibility in the channel shape since the channel shape may be defined(e.g. with a relatively high resolution) in the plane of (each) of thelayers. For example, in this way, it may be possible to fabricate curvedchannels, with the curved channel shape being patterned on each layerand the layers then being stacked together to extend the curved channelinto the third dimension to define the unit cell.

In embodiments (of both the first and second aspects of the invention),an or each individual unit cell may be fabricated from a plurality oflayers. That is, the layered construction described above may be used tofabricate a single unit cell. However, as mentioned above, it is alsocontemplated that a (single) acoustic surface comprising a plurality ofunit cells may be fabricated from a plurality of layers. In this case,each layer may comprise a portion or slice of each of the unit cellswithin the acoustic surface. For instance, in embodiments, the devicemay comprise an acoustic surface comprising a plurality of unit cellsarranged into an array, wherein the acoustic surface is constructed froma plurality of layers that are stacked together to define the unit cellswithin the array. It is also contemplated that only a portion of anacoustic surface of the device may be constructed in this way. Forexample, blocks of unit cells may be fabricated as a plurality oflayers, and individual blocks of unit cells may then assembled togetherto define an acoustic surface.

The thickness(es) of the layers may generally be set as desired (atleast within practical limits), e.g. depending on the desired resolutionand operating frequency and/or overall thickness (height) of the unitcell. For instance, depending on the application, the thickness of(each) layer may be less than 500 microns, such as less than 300microns, or less than 100 microns. The layers within a given unit cellmay each have the same thickness or may have different thicknessesrelative to each other (providing an additional means for designing thechannel structure). It will be appreciated that the unit cells maycomprise any number of layers as desired. For example, typically, theunit cells may comprise 5, 6, 7, 8, 9, 10, 11, 12 or more layers stackedtogether. In order to provide a planar (e.g. flat) surface, typicallyall of the unit cells will have the same thickness, and may thereforeeach comprise the same number of layers. However, various arrangementsare possible.

Because the layered construction may allow relatively higher resolution(e.g. smaller) features to be defined, it will be appreciated that thelayered construction may allow significant flexibility in the type ofstructures that can be generated. Thus, it is possible to fabricate inthis way a large number of different types of unit cells encodingdifferent time delays. For instance, as mentioned earlier, a set of unitcells may be fabricated spanning the phase delay range 0 to 2π inuniform intervals. However, the layered construction may allow forfabricating unit cells having essentially arbitrary phase delays. Thismay therefore open up more possibilities for non-uniform quantisation,wherein each unit cell is designed to encode a specific desired phasedelay value e.g. that substantially matches a desired analogue phasedelay value required to generate a given output. Thus, the unit cellsformed in this way may provide a fuller ‘palette’ of time (phase) delayvalues that can be used to reproduce a desired acoustic field (e.g.allowing for a higher bit rate quantisation of the phase delay valueswithin the device).

In general, where a layered construction is used for the unit cells, thelayers may be stacked together in various suitable ways. For instance,various suitable wafer bonding techniques may be used to bond the layerstogether including (among others) e.g. fusion/direct bonding, anodicbonding, eutectic bonding and/or adhesive bonding. However, the layerswithin each unit cell (or surface) need not be (directly) bondedtogether and in some cases the layers may be spaced apart e.g. and heldwithin a mounting structure or frame.

The layers within each unit cell (or each block or array of unit cells)may be stacked together to define a fixed structure. For example, thelayers within a single unit cell (or a block of unit cells) may be fixedtogether to define a pre-configured unit cell (or block) for use withthe first main embodiment of the first aspect of the invention.Similarly, in embodiments of the second aspect, a fixed array may begenerated wherein the stack of layers define a fixed (pre-determined)spatial array distribution.

However, the layers within each unit cell (or within a surface) need not(all) be fixed together. That is, in embodiments, at least some of thelayers are able to move (e.g. slide) relative to the other layers. Inthis way, it is possible for the unit cells and/or array of unit cellsto be re-configured. For instance, the width of the channel and/or theopening of the channel for a unit cell may be controlled byappropriately moving (at least some of) the layers relative to eachother. Thus, in embodiments, where the structure of a unit cell isdefined by a plurality of layers stacked together, the unit cell may bere-configured to introduce a different time (or phase) delay by movingor sliding the layers relative to one another to change the structure ofthe unit cell. It will be appreciated that this approach generally lendsitself to electronic and/or computer control. The device may thusfurther comprise one or more electrodes or electrode layers forproviding control signals for moving the layers. For example, one ormore piezoelectric actuators may be provided for controlling therelative positions of the layers and hence the shape of the channel. ACMOS chip can then be used to control the voltage on each actuator, thuscontrolling e.g. each of the unit cells in the array. In this way, itmay be possible to realise a highly dynamic and miniaturised spatialsound modulation device.

Also provided is a method of fabricating a unit cell or array of unitcells from a plurality of layers. For instance, typically, the methodmay comprise: determining a respective pattern for each of a pluralityof layers required to fabricate a given unit cell or array of unitcells; forming a plurality of layers incorporating the determinedpatterns; and stacking the plurality of layers together to form the unitcell or array of unit cells. The layers may be formed using any suitabletechniques. For instance, as explained above, the layers may suitably beformed using existing MEMS fabrication techniques. For example, thelayers may be patterned by using laser cutting, wet etching, orphotolithography techniques to create one or more openings in thelayers.

At least according to embodiments of the first aspect described above,the array of time delays defined by the plurality of unit cells may bere-configured to vary the spatial delay distribution in order togenerate different acoustic outputs. However, various other embodimentsare also contemplated for adjusting the device to generate differentacoustic outputs.

Accordingly, from a third general aspect of the present invention, thereis provided device for manipulating an incident acoustic wave togenerate an acoustic output comprising: a plurality of unit cellsarranged into an array, at least some of said unit cells beingconfigured to introduce time delays to an incident acoustic wave at therespective positions of the unit cells within the array of unit cells,such that said plurality of unit cells define an array of time delays tothereby define a spatial delay distribution for manipulating an incidentacoustic wave to generate an acoustic output, wherein the device isadjustable in order to generate different acoustic outputs.

As described above, the device may be adjusted by adjusting the array oftime delays provided by the unit cells. However, it is also contemplatedthat the device (as a whole) may be adjusted to generate differentacoustic outputs. For instance, different areas of the device (array)may be configured to perform acoustic manipulations. By moving thedevice (e.g. relative to an acoustic source) so that different areas ofthe device are ‘illuminated’ by the incident acoustic wave the acousticoutput of the device can thus be varied. For example, the device may bephysically rotated (rotatable) between a plurality of differentpositions wherein the portion of the array of unit cells that isilluminated at each position is associated with a different acousticmanipulation. An apparatus may thus be provided comprising an acousticmanipulation device substantially as described herein and an adjustableplatform for moving e.g. rotating the device in order to generatedifferent acoustic outputs. Other arrangements would of course bepossible. For instance, the device need not be rotatably moved but maybe caused to move in any suitable and desired manner.

Similarly, instead of moving the device itself e.g. relative to anacoustic source, a “masking” or “covering” element may be used toselectively cover certain areas of the device (and leave other regionsexposed) so as to vary the acoustic output. This will be explainedfurther below.

Also, it will be appreciated that the device of the third aspect maycomprise any (or all) of the features described above in relation to thefirst and second aspects. For instance, the unit cells may generallycomprise unit cells of any of the types described above. For example,the unit cells may comprise either pre-configured or re-configurableunit cells. Furthermore, the unit cells may either be fabricated using alayered construction technique or otherwise.

In general, the devices described herein may be operated at a range ofdifferent frequencies. However, in embodiments, the device may besubstantially optimised or configured for operation at a certainoperating wavelength, λ₀. For instance, and in general, and as describedabove, according to any of the aspects described herein (and both thefirst and second main embodiments of the first aspect), the unit cellswithin the device may be configured so as to introduce a desired phasedelay, or phase delays, for incident acoustic waves at a certainoperating wavelength, λ₀. For example, the dimensions of the unit cells,and the structures thereof, may be designed so as to introduce a desiredphase delay at the particular operating wavelength or wavelengths forwhich the device is optimised. Furthermore, the unit cells may bedesigned to have a relatively high transmission (e.g. substantially100%) at the operating wavelength, λ₀.

Of course, the device may still be used at wavelengths other than theintended design wavelength. However, in that case the phase delaysintroduced by the unit cells will generally be different. For example, aunit cell that is configured to introduce a first phase delay at a firstoperating frequency may introduce a different phase delay at a secondoperating frequency. However, there may be another unit cell thatintroduces the same first phase delay at the second operating frequency.Based on knowledge of the operating frequency, it may therefore bepossible to select the appropriate unit cells for use at that frequency,even where that frequency is not the frequency for which the unit cellswere originally configured. For instance, a suitable lookup table may beconstructed and used to associate the unit cells with the appropriatephase delays at the selected frequency or frequencies. Alternatively,the unit cells may be configured to introduce a fixed time delay that issubstantially independent of frequency. A desired phase delay can thenbe achieved by using the appropriate time delay for the selectedfrequency or frequencies. Alternatively, cells designed to operate atdifferent frequencies may be arranged together in a “block” which willwork over the range of frequencies defined by the single cells.

In some embodiments, the device may be designed or configured foroperation in the ultrasonic range. For instance, the device may besubstantially optimised or configured for operation at an operatingwavelength, λ₀, within the ultrasonic range. For example, the device maybe optimised or configured for operation at a frequency of about 40 kHz.However, it will be appreciated that the device may suitably be designedor configured for operation in any frequency range and the operatingfrequency or frequencies may e.g. be in the audible frequency range (forinstance, to manipulate a loudspeaker output) or in the MHz range (forinstance, where the device is intended to be used in a liquid medium).

In some embodiments, the unit cells may be configured to transmitacoustic waves substantially only at the operating wavelength, λ₀, forwhich the device has been designed to operate at. That is, thetransmission of the incident acoustic wave may be substantially zero atwavelengths other than the operating wavelength.

Preferably, however, the device may be configured to also operate atfrequencies other than the designated operating wavelength, λ₀. Thus, itis contemplated that although the device and/or unit cells may beoptimised or configured for operation at a particular operatingwavelength, λ₀, by appropriate design of the unit cells, the unit cellsmay also be configured to transmit and/or manipulate acoustic waves atwavelengths other than the designated operating wavelength, λ₀.

That is, although the unit cells may be designed for operation at aparticular operating wavelength, this does not mean that the devicecannot be used at other wavelengths.

In particular, where the unit cells are optimised or configured foroperating at an operating frequency, f₀ (=c/λ₀), the unit cells maysuitably be designed to also transmit acoustic waves at all frequencies,f_(j), satisfying the relationship: f_(j)=f₀−jc₀/L_(ef), wherein j is aninteger, c₀ is the speed of sound through the unit cell and L_(eff) isthe effective path length through the unit cell that determines thephase delay introduced by the unit cell (φ=e^(ikLeff) for an incidentacoustic wave of wavenumber k). In this way, a device having amulti-frequency response may be provided.

Furthermore, it has been found that the operating frequency andbandwidth of a unit cell may generally be related to the transmission ofacoustic waves through the unit cell (essentially because the unit cellsmay act as resonant structures). That is, the transmission (orreflection) efficiency of each of the unit cells may provide a furtherparameter for controlling or adjusting the output of the acousticmanipulation device, particularly to provide a different frequencyresponse. Thus, in embodiments, instead of configuring a unit cell witha relatively high (e.g. substantially 100%) efficiency at the operatingfrequency, the transmission or reflection efficiency of the unit cell(s)may be selected or adjusted in order to vary the acoustic output of thedevice. For instance, each of the unit cells may have an associatedamplitude (e.g. or intensity) value representing the relative amplitude(e.g. intensity), or change in amplitude (e.g. intensity), introduced bythat unit cell to an incident acoustic wave of a particular frequency(e.g. at the operating frequency of the device) passing through the unitcell. Thus, by appropriately selecting or configuring the amplitude(e.g. intensity) values for a unit cell it is possible to change theacoustic manipulation provided by that unit cell. For example, theamplitude (e.g. intensity) value for a unit cell may be selected orconfigured e.g. to increase or optimise the operating bandwidth for thatunit cell.

It will be appreciated that this amplitude (e.g. intensity) optimisationmay generally be performed according to any of the aspects andembodiments described herein. For example, as well as (or instead of)re-configuring the array of time delays defined by the plurality of unitcells, it would also be possible to adjust the device by re-configuringthe amplitude (e.g. or intensity) values associated with the unit cells.Accordingly, each of the unit cells may have an associated amplitude(e.g. or intensity) value representing the relative amplitude (e.g.intensity), or change in amplitude (e.g. intensity), introduced by thatunit cell to an incident acoustic wave, and wherein the array of unitcells may be re-configured to vary an amplitude (e.g. intensity)distribution of the array of unit cells in order to generate differentacoustic outputs. That is, amplitude modulation may be performed acrossthe surface of the array.

Alternatively, or additionally, a multi-frequency response may beprovided by incorporating different types of unit cell that areconfigured for operating at different wavelengths into the array. Thus,the device may in embodiments be configured to operate at a plurality ora range of operating wavelengths.

For instance, in some embodiments, the array of unit cells may include aplurality of different unit cells that are configured to operate atdifferent frequencies. For example, the array of unit cells may comprisea first set of unit cells configured to introduce a first set of desiredphase delays to an incident acoustic wave at a first operatingwavelength (i.e. to perform a first operation, or acoustic manipulation,at the first operating wavelength) and a second set of unit cellsconfigured to introduce a second of desired phase delays to an incidentacoustic wave at a second operating wavelength (i.e. to perform a secondoperation, or acoustic manipulation, at the second operatingwavelength). The first and second set of unit cells may generally bearranged to perform the same operation (but for different incidentfrequencies). However, it would also be possible for the first andsecond set of unit cells to perform different operations, so thatdifferent frequency incident acoustic waves were manipulated indifferent ways (e.g. like in a prism). It will be appreciated that thearray of unit cells may comprise any number and type of unit cells.Thus, the array of unit cells may also generally comprise a third orfurther set of unit cells configured to introduce a third or further setof desired phase delays to an incident acoustic wave (or to perform athird or further operation or acoustic manipulation) at third or furtheroperating wavelengths.

In such cases, a selective “masking” or “covering” element (e.g. in theform of a masking layer comprising a number of open elements (areas)that allow acoustic waves to pass through and a number of closedelements (areas) that prevent acoustic waves from passing through) maybe provided that is configured to be able to selectively cover at leastsome of the unit cells. For instance, the masking element may beconfigured to selectively cover one or more of the different sets ofunit cells (whilst leaving one or more other of the sets of unit cellsopen). For example, where the array of unit cells comprises a first andsecond set of unit cells as described above, the masking element may beselectively positioned (or positionable) to cover either the first orsecond set of unit cells (whilst leaving the other set of unit cellsopen). In this way, the device can be effectively and rapidly switchedbetween different operating frequencies. For example, where the maskingelement covers the first set of unit cells, so that only the second setof unit cells are open, the device will be optimised for operating atthe second wavelength, and vice versa. Where three or more sets of unitcells are provided, the masking element may be operable to selectivelycover multiple sets of unit cells at any given time, e.g. all but one ofthe sets of unit cells.

(The use of such a selective masking element may also be used moregenerally to adjust the operation of the device e.g. in accordance withthe third aspect described above. For instance, the array of unit cellsmay generally comprise a plurality of different types of unit cells andthe masking element may be used to select a subset of types of unitcells. That is, this approach is not limited to frequency switching butmay more generally be used for switching between other operations. Thus,the array of unit cells may comprise a first set of unit cellsconfigured to perform a first operation (e.g. at a first frequency) anda second set of unit cells configured to perform a second operation(e.g. at a second frequency). For instance, in embodiments, the firstset of unit cells may perform a first acoustic manipulation at the firstfrequency whereas the second set of unit cells perform a second acousticmanipulation at the same first frequency. By using the masking elementto select between the first and second sets of unit cells the device canthus be switched between the first and second acoustic manipulations.Naturally, the array of unit cells may also comprise third or furthersets of unit cells allowing the device to be switched between furtheroperations. Furthermore, it will be appreciated that the array of unitcells may generally comprise any number and types of unit cells. So, forinstance, with an appropriate selection of unit cell types, it would bepossible to provide a device that is capable of switching betweendifferent operating frequencies and different operating functions.)

In other cases, a block may be formed by N different types of cells,each optimised to operate in a narrow-band centred on a single differentfrequency e.g. (f₁, Δf₁) for the first cell, (f₂, Δf₂) for the secondcell . . . (f_(N), Δf_(N)) for the N^(th) cell. In these cases, thecells may be designed to be much smaller than the smallest of theassociated wavelengths (i.e. λ₁, λ₂, λ₃ etc.), and their frequencies tobe sufficiently close together (e.g. with f₂−f₁<Δf₁), so that the wholeblock covers a larger bandwidth.

The device according to the present invention may thus be used tomanipulate incident acoustic waves at the operating wavelength, λ₀, forwhich the, or at least some of the, unit cells have been optimised orconfigured. However, in embodiments, the device may also oralternatively be used to manipulate incident acoustic waves at otherwavelengths. It will be appreciated that at other wavelengths the devicemay no longer be optimised for transmission and/or phase delay.

Where the device is optimised or configured for operation at anoperating wavelength, λ₀, at least some of, or each of, the plurality ofunit cells may have a dimension within the array, e.g. at the surface ofthe device or in the plane of the array (i.e. the surface or plane uponwhich an acoustic wave to be manipulated is incident), of half theoperating wavelength (i.e. λ₀/2), or smaller. It has been found thatlimiting the size of the unit cells within the array to this dimensionhelps to provide better spatial resolution for generating or recreatingdesired acoustic waves. Where the dimension of the unit cells is halfthe operating wavelength (i.e. λ₀/2) or smaller, the device may alsosuitably be used for frequencies less than the operating wavelength atwhich the unit cells were optimised or configured for operating at. Onthe other hand, operating the device at frequencies higher than theoperating wavelength (i.e. greater than λ₀) may result in the appearanceof acoustic field artefacts and a loss of accuracy.

In general, it is contemplated that where the lateral dimension of theunit cells (i.e. the dimension of the unit cells that defines the arrayof unit cell) is fixed at some value, L, the device may suitably be usedat a or all frequencies below a maximum frequency, f_(max)=c/2L.

It is contemplated that the device may be used in a single or monofrequency operation. However, it is also contemplated that the devicemay be used for “broadband” operation. For instance, a limited band offrequencies around a central operating frequency may be passed to thedevice. By appropriate design of the unit cells, for example such thatthe effective path length (and hence time delays) do not depend onfrequency, at least in the frequency range of operation, the unit cellsmay transmit across the range of frequencies. The array of unit cellsmay be designed so as to effectively average the frequency response ofthe individual unit cells to allow the device to work over the frequencyrange. Alternatively, the different frequency response(s) of the unitcells may be exploited to produce a frequency dependent acoustic output.For example, the device may be configured to manipulate an incidentacoustic wave containing a range of frequencies to generate a firstacoustic output associated with a first frequency and a second acousticoutput associated with a second frequency and so on. The device may thusbe used to effectively split the different frequency components of theincident acoustic wave.

As described above, the unit cells may typically be configured for(optimal) operation at a single operating frequency. However, where alayered construction is used for the unit cells, this may allow for theimplementation of various perturbation methods for increasing thefrequency band where each unit cell (or array of unit cells) canoperate. For instance, as described above, each unit cell may generallycomprise a channel that introduces a time delay to an incident acousticwave. By introducing vibrations or perturbations in the layers of a unitcell that cause corresponding variations in the length and/or shape ofthe channel it may be possible to maintain the transmission efficiencyfor the unit cell over a wider range of frequencies with minimal changesin the output. Typically, these will be relatively small (low amplitude)and high-frequency vibrations. Thus, in embodiments, where a unit cell(or array of unit cells) comprises a plurality of layers that arestacked together to define the structure of the unit cell (or array ofunit cells), at least some of the layers may be caused to vibrate (orotherwise move continuously to and fro) relative to each other to adjustthe structure of the unit cell. For instance, in some examples, at leastone of the layers may be mechanically moved at a frequency higher thanthe operating acoustic frequency. In this case, the perturbationfrequency may be at least twice as large as the highest acousticfrequency in order to help reduce artefacts. However, even if it is notpossible or practical to mechanically move the layer(s) at thisfrequency, there may still be some benefits in using lower perturbationfrequencies e.g. in terms of the fidelity of the acoustic output.

The array may be provided in the form of one or more layers of unitcells. A layer of unit cells (which may also be referred to as an“acoustic surface”) preferably comprises a layer having a single unitcell thickness. The plurality of unit cells may thus be arranged in alayer (acoustic surface). The arrangement of the unit cells within a oreach layer (acoustic surface) may be substantially flat ortwo-dimensional. That is, a or each layer (acoustic surface) may besubstantially planar, with the array of unit cells within each layerarranged in a plane. However, it is also contemplated that thearrangement of unit cells within a or each layer (acoustic surface) neednot be flat, and a layer may define a curved surface. For instance, theunit cells within the layer (acoustic surface) may be mounted on, orotherwise arranged to form, a curved surface. The curved surface maygenerally be convex or concave or otherwise profiled. Thus, thesurface(s) of the device may generally be planar or curved.

In embodiments, the device may comprise a stack of two or more layers(acoustic surfaces), each layer (acoustic surface) comprising an arrayof unit cells and/or time delay values. That is, a device may be builtup by stacking multiple layers of arrays of unit cells of the typedescribed above. Where the device comprises a stack of two or morelayers (acoustic surfaces), each layer (acoustic surface) may have adifferent spatial delay distribution and/or a different spatialconfiguration of unit cells.

Because time delays are additive, appropriately stacking multiple layers(acoustic surfaces) together may allow more complex transformations orcombinations of transformations to be performed. For example, bystacking a ‘steering’ layer (surface) with a ‘focussing’ layer(surface), it is possible to perform ‘steered focussing’.

Considered another way, multiple devices may be stacked together inorder to build more complex devices.

In embodiments, the structure of the unit cells may be configured ordesigned to facilitate stacking. For instance, the unit cells may bedesigned to have nearly 100% transmission (or reflection) at theoperating wavelength(s). (However, it is also contemplated in otherembodiments, as mentioned above, that the transmission (reflection)efficiency of the unit cells may be controlled e.g. in order to adjustthe frequency response of the unit cells.)

The unit cells may generally be impedance matched with the ambientmedium to reduce reflections. For instance, where the device isoperating in air, the unit cells may be impedance matched with air.However, it will be appreciated that the device may be operated in otherambient media, including liquids such as water, depending on theapplication.

It will be appreciated that using substantially flat or two-dimensionallayers may facilitate stacking of the layers (acoustic surfaces).However, it will be appreciated that curved layers (surfaces) mayequally be stacked, and that the device may therefore comprise a stackof two or more curved layers (surfaces), or a stack comprising a mixtureof flat and curved layers (surfaces). In this way, the orientation ofthe unit cells may also contribute to the manipulation of the incidentacoustic waves. By using appropriately shaped surfaces it may bepossible to reduce the number of types of unit cells that are requiredto produce a given field with a desired precision.

The devices of the present invention, according to any aspects, aregenerally arranged to manipulate an incident acoustic wave. Generally,this is a spatial manipulation of the incident acoustic wave, i.e. thedevice acts to spatially modulate, shape, or otherwise control theincident acoustic wave. The manipulated acoustic wave is then providedas the acoustic output of the device.

For example, in embodiments, the spatial delay distribution of the arraymay be configured so as to focus an incident acoustic wave. In anotherexample, the spatial delay distribution of the array may be configuredso as to steer or direct an incident acoustic wave. As a furtherexample, the spatial delay distribution of the array may be configuredso as to introduce a phase delay of about π radians (a “half-wave”delay) for incident acoustic waves at a particular frequency. However,it will be appreciated that by suitably varying the spatial delaydistribution of the device, it is possible to realise a great number ofdifferent manipulations, so as to be able to generate or reproduceessentially arbitrarily complex acoustic outputs. In embodiments, thedevice according to any of the aspects described herein may be used incombination with an acoustic source in order to manipulate the acousticwaves generated by the acoustic source. It will be appreciated that themanipulation performed by the device may be essentially independent ofthe acoustic source. That is, the manipulation of the incident acousticwave by the device is generally controlled by the distribution of timedelays across the device, and not by the form of the incident acousticwave. Advantageously, this means that the device does not need to drawany power from the acoustic source and that the device may thus bere-configured independently of the acoustic source. This separation ofthe acoustic source from the manipulation may help to simplify the powerrequirements for the acoustic source and/or for the device. By contrast,in conventional phased transducer arrays, because the sound modulationis performed by the transducers themselves, any switching of thetransducers to re-configure the acoustic wave results in a loss ofacoustic power. For instance, typically around 10-20% of the acousticpower may be lost when re-configuring a phased transducer array. Thepower requirements for a conventional phased transducer arrays aretypically relatively complicated requiring a large number of high powerchannels, which can be expensive and difficult to control.

Because the manipulation may be essentially independent of the acousticsource, the form of the incident acoustic wave and hence of the acousticsource does not particularly matter and the devices according to thepresent invention may generally be configured to receive and manipulateany incident acoustic wave.

For instance, in embodiments, the devices according to the presentinvention in any of its aspects may be used to manipulate an acousticwave that is incident normally to a surface of the device and issubstantially uniform over the surface of the device.

In this way, the power requirements for the acoustic source can bededicated solely to providing acoustic wave strength, and need notperform any manipulation, which can be achieved solely using the devicesaccording to the present invention. Thus, an assembly may be providedcomprising an acoustic source for generating such acoustic wavescombined with a device substantially as described herein in relation tothe present invention.

However, in other embodiments, the devices may be used with arbitrary orpre-existing acoustic sources. For instance, the devices may beretro-fitted or added to existing acoustic sources in order to provide adesired manipulation. The device may thus be provided with a sleeve orother member for facilitating mounting of the device around the acousticsource. In this case, the devices according to the present invention maybe used to further manipulate acoustic waves that have already beenshaped. For example, a device according to the present invention may beused to further manipulate (e.g. steer or focus) an acoustic waveproduced by a directional loudspeaker. In this way, the final acousticoutput may be determined in part both by the acoustic source and thedevice according to the present invention.

In other embodiments, the devices of the present invention may be usedin combination with a suitable sensor or detector as part of an imagingor sensing assembly. For instance, the device may be used to receive orsense an incident acoustic wave, and to direct the acoustic output ontoor towards the sensor or detector for recording and/or analysis. In thiscase, the arrangement of time delays of the unit cells within the arraymay be appropriately selected depending on the desired application in asimilar manner to that described above. For instance, the device may beconfigured to focus the incident acoustic wave onto a sensor ordetector. However, it also contemplated that the device may beconfigured to perform various other manipulations depending on theapplication in order to help detect a desired property. For instance,the device may be configured to sum the contributions of the incidentacoustic wave(s) at different spatial positions in the device accordingto the spatial delay distribution of the device. The device may thus beconfigured to act as a radar, or a sonar, wherein the device acts tocapture acoustic waves from a specific position and/or direction and totransmit the capture acoustic waves onto a (fixed) sensor or detector.

The array of unit cells may generally comprise any suitable number ofunit cells and the unit cells may generally be arranged within the arrayin any suitable manner. The array need not be a regular array and ingeneral the unit cells may be arranged relative to one another in anysuitable and desired arrangement. However, in embodiments, the array maybe a two-dimensional array of M×N unit cells where M and N may eachindependently comprise any integer value. For instance, the values of Mand/or N may each be selected from the list comprising 1, 2, 3, 4, 8,16, 24, 100. As mentioned above, some of the elements of the array maycomprise empty cells. In some embodiments, the array may comprise aregular rectangular or square array of unit cells. For example, thearray may comprise an array of 16×16 unit cells, which may e.g. besuitable for creating a twin-trap. As another example, the array maycomprise an array of 24×24 unit cells which may e.g. be suitable forcreating a tactile focal point. As a further example, the array maycomprise an array of 100×100 cells which may e.g. be suitable forcreating three-dimensional figures made of levitated objects. It will beappreciated that the unit cells in the array need not be square-packedand that the array of unit cells may be arranged according to othersuitable packing methodologies, such as a random or hexagonal packing.In embodiments, the array may comprise a three-dimensional arrangementof unit cells, for example, a plurality of unit cells arranged in one ormore separate layers.

In general, the devices described herein may be configured to operateeither in transmission or reflection. That is, the device may beconfigured so that when an incident acoustic wave is provided on a firstside of the device, acoustic waves travel through and out of the deviceto provide an acoustic output on the opposite side of the device (i.e.in transmission). However, the device may alternatively be configured sothat the acoustic output is provided on the same side of the device ontothe incident acoustic wave is provided (i.e. reflection). For instance,when the unit cells comprise a central channel, the channel may be open,and extend between opposite sides of the unit cell so that acousticwaves are transmitted from one side of the device to the other.Alternatively, the channel may be closed at one end to cause acousticwaves to be reflected. It is also contemplated that in some examples thedevice may be used to transfer (incident) evanescent waves into asurface.

The device may comprise an acoustic waveguide for guiding the incidentacoustic wave onto or towards the array of unit cells. In this way, themanipulation of the incident acoustic wave may be physically (as well asconceptually) removed from the acoustic source. This may be useful whenit is desired to provide the acoustic output to a particular areawithout having to install a potentially bulky acoustic source in thatarea (i.e. for aesthetic or space-saving reasons).

The devices described herein may be provided as ‘stand-alone’ acousticmanipulation devices. However, it is also contemplated that the devicesdescribed herein may be incorporated as part of a larger structure.Accordingly, from another aspect there is provided a structurecomprising a device substantially as described in relation to any of theaspects or embodiments herein. For example, a device comprising aplurality of unit cells may be provided on an outer surface of astructure to provide the structure with the ability to spatiallymodulate acoustic fields. This may be advantageous for instance forvarious noise control applications. For instance, an acoustic modulationdevice of the type described herein may suitably be incorporated intobuilding materials such as house bricks or insulation for the purposesof noise reduction. The device may generally be provided either as alayer on top of the existing structure or formed integrally with thestructure.

From another aspect there is provided a method of generating a spatiallymodulated acoustic field comprising: generating an acoustic input wave;and passing the acoustic input wave through a device substantially asdescribed herein in relation to the present invention in any of itsaspects or embodiments so as to manipulate the acoustic input wave togenerate a desired acoustic output.

From a further aspect there is provided a method of acoustic imaging orsensing comprising: passing an acoustic input wave through a devicesubstantially as described herein in relation to the present inventionin any of its aspects or embodiments so that the device manipulates theacoustic input wave to generate an acoustic output; and detecting theacoustic output at a sensor or detector.

The methods according to these aspects may further comprisere-configuring the device to generate a different acoustic output. Forinstance, the device may be re-configured according to either of thefirst or second main embodiments of the first aspect described above.

The aspects described above primarily relate to the configuration of thedevice. However, also presented herein are methods of determining howthe device may be configured (or re-configured) in order to generate adesired acoustic output.

In relation to the first aspect, these methods may thus be used todetermine which, or how many, pre-configured unit cells need to bemanufactured and/or provided to a user in order to meet the user'srequired specification. Similarly, the methods may be used to determineand generate electronic control signals required for controlling anarray of re-configurable unit cells. Generally, the methods involvedetermining which time or phase delays, i.e. which time or phase delayvalues need to be introduced at which positions within the array inorder to generate a desired acoustic output with a desired accuracy.

Thus, from a further aspect, there is provided a method of designing orconfiguring a device for manipulating acoustic waves comprising aplurality of unit cells arranged into one or more layers each layercomprising an array of unit cells, at least some of the unit cells beingconfigured to introduce time delays to an incident acoustic wave at therespective positions of the unit cells within the array(s) of unitcells, such that the plurality of unit cells define an array of timedelays to thereby define a spatial delay distribution for manipulatingan incident acoustic wave to generate an acoustic output, the methodcomprising: determining a quantised delay distribution of a desiredanalogue acoustic field containing a set of discrete pairs of time delayvalues and spatial positions representing the distribution of timedelays required in the device for generating the desired analogueacoustic field; mapping the quantised delay distribution of the desiredanalogue acoustic field to the positions and time delay values of theunit cells for the device; and selecting the time delays of the unitcells for (or within) the device based on the mapping.

Generally, the device that is being designed or configured according tothis aspect may be a device of the type described above in relation tothe previous aspects and embodiments of the present invention. Forinstance, the device may be a re-configurable device as described inrelation to the first aspect. However, it will be appreciated that thesemethods may also advantageously be used to design or configure a fixeddevice wherein the array of time delays is not re-configurable in use.

For instance, in relation to the second aspect, these methods may beused to determine the required time delay values for each of the unitcells within the array (which may then in turn be used to determine therequired structure for each of the layers that are then used to form thearray).

The desired analogue acoustic field may be a theoretical or simulatedfield, or may be a real acoustic field. In the latter case, the realacoustic field may be sampled in order to determine a sampled phasedistribution, and the sampled phase distribution may then be quantisedappropriately in order to determine the distribution of time delays orphase delays required at the device to reproduce the field. The realacoustic field may be sampled at a certain distance from the device, andthe sampled field may thus need to be first mapped onto the surface(s)of the device in order to determine the time delays and positionsrequired at the device for reproducing the acoustic field. This mappingmay be done, for example, using acoustic holography techniques. In othercases, or where the acoustic field is theoretical or simulated, theacoustic field may already be sampled at a surface of the device.

In any case, the result of the quantisation step is a quantised delaydistribution containing a set of pairs of discrete spatial positions andtime delay values representing the (discrete) time delay that isrequired at each discrete position in the device in order to bestreproduce or generate the desired analogue acoustic field. Thus, theresult of the quantisation step is that the desired analogue field iseffectively quantised in both the spatial and time delay domains. Thequantised phase distribution i.e. the determined pairs of spatialpositions and time delay values may then be mapped onto the unit cellsof the device. It will be appreciated that in order to reproduce thedesired acoustic field it may be necessary to determine the phase delaysrequired at each discrete position on the device. The quantisation stepmay thus be used to determine a quantised phase distribution of thedesired analogue acoustic field containing a set of discrete pairs ofphase delay values and spatial positions representing the distributionof phase delays required at the device (e.g. at the surface or in theplane of the device) for generating the desired analogue acoustic field.The quantised phase distribution may then be mapped to the phase delayvalues of the unit cells. The mapping may therefore take into account,or may determine the appropriate, frequency or frequencies of operationrequired to generate the desired analogue acoustic field.

During the quantisation, the amplitude/intensity for each of the unitcells may be fixed (e.g. at unity) so that the quantisation is performedsolely in the spatial and time delay domains. However, it would also bepossible to perform quantisation in the amplitude or intensity domain,as well as (or even instead of) in the time delay domain. Thus, in somecases, the quantisation may involve determining a quantised delaydistribution of a desired analogue acoustic field containing a set ofdiscrete pairs of time delay and/or amplitude or intensity values andspatial positions representing the distribution of time delays and/oramplitudes or intensities required in the device for generating thedesired analogue acoustic field; mapping the quantised delaydistribution of the desired analogue acoustic field to the positions andtime delay and/or amplitude or intensity values of the unit cells forthe device; and selecting the time delays and/or amplitude or intensityvalues of the unit cells for (or within) the device based on themapping.

Accordingly, from another aspect, there is provided a method ofdesigning or configuring a device for manipulating acoustic wavescomprising a plurality of unit cells arranged into one or more layerseach layer comprising an array of unit cells, at least some of the unitcells being configured to introduce time delays to an incident acousticwave at the respective positions of the unit cells within the array(s)of unit cells, such that the plurality of unit cells define an array oftime delays to thereby define a spatial delay distribution formanipulating an incident acoustic wave to generate an acoustic output,the method comprising: determining a quantised delay distribution of adesired analogue acoustic field containing a set of discrete pairs ofamplitude or intensity values and spatial positions representing thedistribution of amplitude or intensity values required in the device forgenerating the desired analogue acoustic field; mapping the quantiseddelay distribution of the desired analogue acoustic field to thepositions and amplitude or intensity values of the unit cells for thedevice; and selecting the amplitude or intensity values of the unitcells for (or within) the device based on the mapping.

Preferably, the quantisation is performed (at least) in the spatial andtime delay domains.

In some cases, the time delays of the unit cells for (or within) thedevice may be selected directly based on the mapping of the quantiseddelay distribution to the positions and time delay values of the unitcells for the device. For instance, where it is possible to fabricateunit cells having essentially arbitrary time delay values, the timedelay values for the unit cells may be set to the (closest) appropriatevalue based on the mapping.

However, it will be appreciated that the resolution of the desiredanalogue acoustic field, or particularly the resolution of the sampling,where such sampling is performed, may not match the resolution of thedevice. That is, the desired analogue acoustic field may be sampledand/or quantised at a first resolution in the spatial and/or delaydomains, whereas the dimensions of the device in the spatial and/ordelay domain may be different, in which case a direct or one-to-onemapping to the available positions and/or the available time or phasedelay values may not be possible (so that some approximation or matchingto the closest available value may be required). It will be understoodthat the resolution in the delay domain is essentially determined by thenumber of available time or phase delay values (e.g. the number ofunique quanta).

Thus, in embodiments, the methods of this aspect may comprise a step ofcompressing the quantised delay distribution in the spatial and/or delaydomains in order to generate a compressed delay distribution in thespatial and/or delay domains, wherein the resolution of the compresseddelay distribution in the spatial and/or delay domains is such that thecompressed delay distribution may be mapped directly to the desiredpositions and time (or phase) delay values of the unit cells for (orwithin) the device. It will be appreciated that a compression step mayoccur simultaneously, or as a part of the quantisation step.

The resulting compressed delay distribution may allow for a direct orone-to-one mapping between the elements of the compressed delaydistribution and the desired positions and/or time (or phase) delayvalues for the unit cells within the device. That is, the resolution ofthe compressed delay distribution may match the achievable resolution ofthe device. The mapping step and/or the setting or selecting stepdescribed above may therefore use the compressed delay distribution.

In this way, the mapping from the quantised delay distribution to thedevice may take into account the spatial and/or delay resolution of thedevice. The compression may advantageously be used in two main ways.

In a first main embodiment of this aspect, the compression step may beused to generate a compressed delay distribution suitable for directmapping onto the available unit cells. The desired positions and time orphase delay values in this case may thus be the available positions andtime or phase delay values for a pre-existing set of unit cells. Thatis, the compression may be used to match the delay distribution of thedesired acoustic field to an existing set of unit cells. Thus, where thespatial delay distribution of the acoustic field is provided or sampledat a first resolution, the compression step is effectively used tochange the resolution in order to match the available or existing unitcells.

In a second main embodiment of this aspect, instead of using thecompression to map onto an available set of existing unit cells, thecompression may be used itself to determine or optimise the types ofunit cells that need to be provided in order to generate the desiredanalogue field. In this case, the desired positions and/or time or phasedelay values are effectively determined from the compression step. Forinstance, the compression may be optimised based on the acoustic outputand the accuracy desired by a user in order to manufacture or provide abespoke set of unit cells. Thus, in embodiments, the method may furthercomprise providing as input an accuracy at which it is desired toreproduce the desired analogue acoustic field. The compression algorithmmay then be run to determine the minimum number and type of unit cellsrequired to produce the desired analogue acoustic field with the desiredaccuracy.

The method may thus comprise determining the minimum number of (unique)unit cells and/or time or phase delay values required to reproduce thedesired analogue field with a desired accuracy based on the step ofcompressing the quantised delay distribution. This may be the minimumnumber of time or phase delay values from an existing set of availabletime or phase delay values (i.e. the method may comprise determiningwhich unit cells from an existing set should be used). However, it isalso contemplated that this may be a determination of the absoluteminimum number of time or phase delay values required to reproduce thedesired analogue field with the desired accuracy. That is, the methodmay comprise optimising the number and type of unit cells to provide thesmallest set of unique unit cells or unique time or phase delay valuesrequired. For instance, the method may comprise determining a set of(typically non-uniformly distributed) phase delay values required for aparticular application. A bespoke set of unit cells may thus bemanufactured according to this optimisation or determination.

It will be appreciated that the minimum or optimum number of unit cellsmay alternatively be determined by various other suitable techniques,for instance, with multi-parameter optimisation techniques such as aleast squares minimisation. The methods of this aspect may generally beused to configure or design either a single layer device, or a devicecomprising a stack of layers, each layer comprising an array of unitcells. Thus, the steps of mapping the quantised delay distribution tothe positions and time delays of unit cells within the device maycomprise either mapping the quantised delay distribution to thepositions and time delay values of unit cells within a single layer, ormay comprise mapping the quantised delay distribution to the positionsand time delay values of unit cells within a plurality of layers.Similarly, the step of setting or selecting the time delay values of theunit cells within the device based on the mapping may comprise eithersetting or selecting the time values of the unit cells within a singlelayer, or may comprise setting or selecting the time delay values of theunit cells within each of a plurality of layers accordingly.

Naturally, where the methods are used to configure or design a singlelayer of unit cells, the quantised or compressed delay distribution maybe mapped to the positions and time delay values of the unit cellswithin that layer. Typically, in this case, the mapping will be lossyi.e. some information from the quantised delay distribution will be lostduring the mapping.

On the other hand, where the methods are used to configure or design adevice comprising a stack of layers of unit cells, the compression stepmay be used to decompose the quantised delay distribution into a seriesof levels, each level containing a set of pairs of spatial positions andtime delay values, such that each level may then be mapped to arespective layer of the device. For instance, the compression step mayeffectively decompose the quantised phase distribution into an algebraicsum of the base functions used in the compression algorithm. By using astack of multiple layers, this sum can be performed physically as eachlayer can represent one of the terms (i.e. base functions) of the sum.In this way, with a suitable selection of the base functions (and unitcells), a theoretically lossless or at least low or lower loss mappingmay be achieved by an appropriate stacking of multiple layers together.

Thus, the step of compressing the quantised phase distribution in thespatial and/or time delay domains may comprise generating a compresseddelay distribution comprising two or more (such as three, or four)layers such that the compressed delay distribution may be mappeddirectly to the positions and time delay values of the unit cells withineach of the two or more layers of the device.

In general, any suitable compression algorithm may be used to generatethe compressed delay distribution.

However, the Applicants have recognised that the phase distribution foran arbitrary acoustic field may typically contain relatively sharp edgesor boundaries between different phases. Thus, when compressing the delaydistribution of an acoustic field, it has been found that it isadvantageous to use a compression algorithm that preserves genericfeatures such as edges. In particular, it has been found that wavelettransformations may suitably be used for compressing a delaydistribution representing acoustic phases. For instance, the compressionalgorithm may decompose the delay distribution into a sum of one or morediscrete wavelets (which form the base functions of the decomposition).By way of example, a Haar or Shannon wavelet decomposition may be used.The Haar wavelet (which is essentially a single cycle of a square-wavefunction) may be particularly suitable since it has been found that unitcells may be designed to provide a physical representation of the Haarwavelet function.

Indeed, in general, the techniques described herein may involve unitcells that are constructed or configured based on a compressionalgorithm that may be used in a step of compressing an acoustic phasedistribution in the manner described herein. For instance, the unitcells may be configured based on the basis functions used by thecompression algorithm, e.g. such that they substantially match the basisfunctions. That is, the time or phase delays introduced by the unitcells (or combinations of unit cells) may be configured or selectedbased on the form of the basis functions used in the compressionalgorithm (e.g. the Haar wavelet function, as mentioned above). This mayfacilitate and/or improve the accuracy of the step of mapping thequantised delay distribution onto the unit cells. In particular, byusing unit cells that can be designed to match the basis functions ofthe compression, a lossless or low loss compression may be facilitated,as mentioned above. For instance, the unit cells within the layers of amultilayer device may be used to physically represent the various termsof the Haar wavelet decomposition.

Once the time or phase delay values required for generating the desiredacoustic output (at the desired level of accuracy) have been determinedusing these methods, it is then possible to select, or to set, theappropriate arrangement of unit cells and/or time or phase delays foruse within the array. For example, it is contemplated that the output ofthese methods may be provided as input to a 3D printer for manufacturinga kit of pre-configured unit cells having the required delays.Alternatively, the output of these methods may be used to designsuitable photolithographic masks for forming a plurality of layers thatcan be stacked together to manufacture the unit cells. In otherembodiments, the output of these methods may be provided to a processoror storage device and used to generate a control signal for controllingan array of re-configurable cells.

DESCRIPTION OF THE FIGURES

Various embodiments will now be described, by way of example only, andwith reference to the accompanying drawings in which:

FIG. 1 shows schematically an assembly for modulating sound according tovarious embodiments of the present invention;

FIG. 2 shows schematically a unit cell construction for introducing aphase delay to an incident acoustic wave;

FIG. 3 illustrates how the unit cell construction shown in FIG. 2 may bedesigned to introduce different phase delays to an incident acousticwave;

FIG. 4A shows a perspective view of a unit cell of the type shown inFIGS. 2 and 3, and FIG. 4B shows a set of unit cells configured tointroduce phase delays spanning the range 0 to 2π in discrete intervalsof π/8;

FIG. 5 shows an example of a frame into which a plurality of unit cellsmay be mounted to form an array of unit cells;

FIG. 6 shows the transmission and reflection behaviour of unit cells ofthe type shown in FIGS. 2, 3, 4A and 4B that are configured to introducephase delays of π (upper panel) and 15π/8 (lower panel);

FIG. 7A shows schematically a re-configurable unit cell constructedaccording to some embodiments of the present invention, and FIG. 7Bshows schematically how the phase delay introduced by there-configurable unit cell may be varied by re-configuring the unit cellbetween different positions;

FIG. 8 shows schematically how a multi-layer device may be constructed;

FIG. 9 shows schematically how a focussing transformation may beachieved according to the techniques described herein;

FIG. 10 shows schematically how a steering transformation may beachieved according to the techniques described herein;

FIG. 11 shows schematically how different transformations may becombined according to the techniques described herein;

FIG. 12 illustrates an example of a compression scheme for designing adevice that is capable of reproducing a desired analogue acoustic field;

FIG. 13 illustrates an exemplary compression algorithm that may be usedfor compressing a quantised phase distribution to change the resolutionin the spatial and phase domains to match those of a device according toembodiments of the present invention;

FIG. 14 shows schematically an example of a unit cell constructionwherein the unit cell is formed by stacking a plurality of ‘horizontal’layers together;

FIGS. 15A and 15B show schematically how the layered unit cellconstruction illustrated in FIG. 14 may be used to construct unit cellsthat introduce different phase delays to an incident acoustic wave;

FIG. 16 shows schematically a layered unit cell construction comprisinga three-dimensional channel;

FIG. 17 shows schematically a layered unit cell construction wherein theunit cell is formed by stacking a plurality of ‘vertical’ layerstogether;

FIG. 18 shows schematically how an acoustic surface comprising aplurality of unit cells may be constructed by stacking together aplurality of layers;

FIG. 19 illustrates an example of a non-uniform quantisation that may beachieved using a multi-layered acoustic surface;

FIG. 20 shows schematically an example of a method for increasing thefrequency band where a single acoustic surface can operate by providinga device that can be switched between different operating frequencies byselectively moving a masking element;

FIG. 21 shows a device that can be switched between different functionsby selectively moving a masking element; and

FIG. 22 shows an example of a structure incorporating an acousticmanipulation device of the type described herein.

DETAILED DESCRIPTION

The concepts described herein relate generally to a novel approach forspatially manipulating sound using acoustic metamaterials. Thus, adevice for manipulating acoustic waves (hereinafter, a “sound modulationdevice”) is provided. In particular, a plurality of unit cells eachcapable of encoding a particular time or phase delay, or plurality oftime or phase delays, are arranged together in an array in order toconstruct an acoustic metamaterial layer (or, alternatively, a“meta-surface”). The time delay or phase distribution of the acousticmetamaterial layer may thus be quantised in the spatial domain accordingto the positions and sizes of the unit cells. The spatial distributionof the time or phase delays across the acoustic metamaterial layergenerally determines how an acoustic wave incident on the metamateriallayer will be transformed or manipulated as it passes through andinteracts with the unit cells of the metamaterial layer. The unit cells,or the arrangement of unit cells within the metamaterial layer, may bere-configured for performing various different acoustic transformationsor manipulations.

Various non-limiting examples and embodiments will now be described tohelp illustrate these concepts.

FIG. 1 shows schematically an assembly for modulating sound according tovarious embodiments of the present invention. The assembly comprises anacoustic source 10 which as shown in FIG. 1 may generally comprise atransducer or array of transducers driven in phase and a soundmodulation device 20 for manipulating the acoustic wave generated by theacoustic source 10. The sound modulation device 20 may be positionedover the acoustic source 10 at a certain fixed distance from the planeof the acoustic source 10 so that the acoustic waves generated by theacoustic source 10 are passed towards and through the sound modulationdevice 20. However, it will be appreciated that the sound modulationdevice 20 need not be positioned directly over the acoustic source 10 inthe manner shown in FIG. 1, and in embodiments desirably may not be, solong as the acoustic waves generated by the acoustic source 10 aredirected towards and through the sound modulation device 20. Forinstance, a guiding member or waveguide may be provided for guiding theacoustic waves onto the sound modulation device 20. In this way thepotentially relatively bulky acoustic source 10, and its associatedpower supply, may be kept physically separate from the spatial soundmodulation device 20. This may be desirable for aesthetic reasons, or inview of size and/or space constraints in the region where the output ofthe spatial sound modulation device 20 is to be provided. Also, in thisway a single acoustic source 10 could potentially be used with multiplesound modulation devices at different positions.

It will be appreciated that the acoustic source 10 itself does notitself need to perform any spatial modulation as this functionality maybe completely devolved to the separate sound modulation device 20. Thatis, the spatial sound modulation device 20 can act independently to theacoustic source 10, and can act on the incident acoustic waves inwhatever form they are provided. Thus, the acoustic source 10 maytypically be arranged to generate substantially uniform acoustic wavesnormal to the surface of the spatial sound modulation device 20 so thatthe spatial modulation can be controlled completely by the soundmodulation device 20. In other embodiments, the acoustic source mayalready provide a directional or focussed acoustic wave. In this case,the spatial sound modulation device may perform an additionalmanipulation on the field. In whatever form they are provided, the soundmodulation device 20 acts to shape or otherwise spatially transform ormanipulate the incident acoustic waves in order to generate a desiredacoustic output field 30. By way of example, FIG. 1 shows the generationof a ‘bottle’ type acoustic field 30 suitable for acoustic levitation.However, as explained further below, the spatial sound modulation device20 may be re-configured to generate different acoustic fields 30 asdesired.

The sound modulation device 20 is generally composed of a plurality ofunit cells each capable of encoding a particular time or phase delay.The positions of the unit cells (and their associated time or phasedelays) thus define the spatial delay distribution for the soundmodulation device 20, which is effectively quantised according to thepositions and dimensions of the unit cells. By controlling the positionsand/or delays of the unit cells within the sound modulation device 20,the sound modulation device 20 may be selectively configured to performvarious manipulations or transformations of an incident acoustic wave.For example, the sound modulation device 20 may be configured to steerand/or to focus the acoustic waves. Thus, by changing the positions ofand/or the time delays introduced by the unit cells within the soundmodulation device 20, the spatial delay distribution of the soundmodulation device 20 may be re-configured in order to perform adifferent function.

In embodiments, the sound modulation device 20 may be substantially flatand two-dimensional as shown in FIG. 1. That is, the spatial soundmodulation device 20 may have substantially flat, parallel upper andlower surfaces. However, it is also contemplated that the soundmodulation device 20, or at least an upper or lower surface thereof, maybe curved or profiled. For instance, the upper or lower surface of thesound modulation device 20 may be substantially convex or concave. Inthis way, the shape of the surface may also in part contribute to thetransformation applied to the incident acoustic waves.

In embodiments, the sound modulation device 20 comprises one or morelayers, with each layer comprising a two-dimensional array of unitcells. Each layer may thus be configured to perform a particular spatialmanipulation. Thus, as shown in FIG. 1, the sound modulation device 20may comprise a stack of acoustic metamaterial layers. Although FIG. 1shows a stack of substantially flat layers, it will be appreciated thatcurved or profiled layers may similarly be stacked. Similarly, multiplesound modulation devices 20 may be stacked together such that the outputof one device is provided as input to the next and so on.

The inter-layer separation within the stack may be selected arbitrarilyor e.g. based on the operating wavelength(s) or desired physical size ofthe device. A suitable inter-layer separation may be of the order of thewavelength(s) for which the device is used. For instance, where thedevice is operated at a wavelength λ₀, the inter-layer separation maysuitably be within the range from about λ₀/4 to 2λ₀. For example,suitable inter-layer separations may be λ₀/4, 3λ₀/4 or 5λ₀/4. Generally,the layers should be stacked sufficiently closely together such that thetime delays introduced by the respective unit cells in each layer areadded together as an acoustic wave passes through the layers of thestack.

Because the time delays introduced by each of the layers in a stack maycombine additively, it will be appreciated that stacking multiple layerstogether, as shown in FIG. 1, allows further possibilities forcontrolling the spatial delay distribution of the sound modulationdevice 20 without necessarily increasing the area of the device, or thenumber or number of different types of unit cells within each layer.Stacking multiple layers in this way may thus allow more complextransformations or combinations of transformations to be realised, or tobe realised with fewer unit cells or simpler control. For example, astack of two or more layers may suitably be used to perform focussedsteering, or to form a bottle shaped beam for acoustic levitation.

For instance, the sound modulation device 20 may comprise a first layer21 that is configured to perform a focussing of the incident acousticwaves and a second layer 22 that is configured to steer or otherwiseshape the incident acoustic waves into the desired acoustic wave 30.

Naturally, the sound modulation device 20 may comprise any number oflayers, and any type of layer configured to perform any suitablefunction. Indeed, an advantage of the concepts described herein is thatthe layers may each be re-configured as desired in order to performdifferent operations on the incident acoustic waves (i.e. to generatedifferent desired acoustic output fields).

As discussed above, it will be appreciated that the manipulation of theacoustic wave may advantageously be performed solely by the soundmodulation device 20. That is, the spatial manipulation or modulationmay be independent of and disconnected from the acoustic source 10. Theacoustic source 10 may thus be used solely for generating the incidentacoustic waves, and may typically therefore generate a substantiallyuniform acoustic wave perpendicular to the surface of the soundmodulation device 20. This means that the modulation does not need todraw any power from the power supply of the acoustic source 10. In thisway the power requirements for the acoustic source and the modulationmay be kept relatively simple (or low), e.g. compared to conventionalphased transducer array approaches, and independent from each other.

This is in direct contrast to known approaches utilising phasedtransducer arrays, where the same elements i.e. transducers are used forgenerating the acoustic wave and for shaping it. These known approachestypically require relatively complex and expensive electronics forre-configuring the acoustic output field. Furthermore, any switching orre-configuring of the phased transducer array results in a loss ofpower. Since the spatial sound modulation techniques described in thepresent application allow the sound power to be disconnected from themodulator, the device may have much lower power requirements thatconventional phased transducer arrays. By decoupling the manipulationfrom the acoustic source, the devices according to the present inventionmay also allow for a faster switching or re-configuration thanconventional phase transducer arrays.

There are two main embodiments for allowing the re-configuration of thespatial sound modulation device 20.

In the first main embodiment, the unit cells are each pre-configured toencode a particular, fixed time delay. The unit cells effectivelytherefore become, in isolation, the building blocks of the acousticmetamaterial layers or meta-surfaces, whereby the individual unit cellscan be assembled on-demand into arrays or layers having a desired delaydistribution. For instance, the pre-configured unit cells may beinterconnected together, or inserted into a frame, to form atwo-dimensional array or metamaterial layer. The unit cells may then bereleased, or removed from the frame, and then re-configured into adifferent arrangement to perform a different transformation.

Because each of the unit cells forming the layer or meta-surface ispre-configured to encode a single, specific time delay, the array orlayer of unit cells quantised in both the spatial and time delaydomains. Various spatial delay distributions suitable for generating agreat number of acoustic output fields may be encoded by selecting theappropriate unit cell (i.e. time delay) for each position within thearray or layer. The accuracy at which the sound modulation device 20 cangenerate a desired arbitrarily complex acoustic wave may in general beincreased by increasing the number of unit cells within the array and/ordecreasing the size of the unit cells within the array (i.e. so that thespatial delay distribution is quantised with a higher resolution), or byincreasing the number of different types of unit cells available (i.e.the number of available time delays and hence the resolution of the timedelays) so that the time delay at each position may be chosen to bettermatch that for the desired field.

The unit cells may take various suitable forms so long as they act tointroduce a well-defined time delay to an incident acoustic wave.Generally, the unit cells may be designed to introduce a local phaseshift at least within the range 0 to 2π for a selected operatingfrequency. In order to form a desired acoustic wave with the requiredaccuracy, and in order to avoid spatial aliasing effects, the unit cellsdesirably hold sub-wavelength resolution. The unit cells should also beable to transmit sound effectively with minimal energy losses,particularly where it is desired to stack the unit cells or layers.

For instance, in embodiments, the unit cells may define a centralchannel through which acoustic waves pass from one side of the unit cellto the other. The central channel may further comprise varioussub-wavelength structures or features that act to slow down the acousticwaves and/or increase the effective path length travelled by theacoustic waves through the channel thereby introducing a phase delay.For example, the channels may include a substantially labyrinthine, ormeandered, structure, or a multi-slit, coil, helical, or Helmholtzresonator-type structure. One suitable structure is illustrated by wayof example in FIG. 2 which shows in cross section an example of alabyrinth structure with meanders defined by four bars extending into anopen channel.

The effective path length, L_(eff), for acoustic waves travellingthrough the unit cell is given by L_(eff)=h+ΔL, where h is the height onthe unit cell, and ΔL is the additional path length introduced by thestructure of the unit cell. This additional path length introduces aphase delay φ=e^(ik.Leff), where k is the wavenumber (k=2π/λ) of theacoustic wave.

The shape and/or dimensions of the unit cells may generally be selectedto introduce a desired phase delay for acoustic waves of a particularwavelength. That is, the design of unit cells may be substantiallyoptimised or configured for use with a particular operating wavelength,such that a desired phase delay is provided for incident acoustic wavesat the operating wavelength, λ₀. In embodiments, the device may bedesigned for use substantially only at a single operating wavelength,such that there is little or no response or transmission at otherwavelengths. In other embodiments it is contemplated that the device maybe designed for use with a range of wavelengths, such as a range ofwavelengths around a central operating wavelength. It is alsocontemplated that the device may be configured to operate at a number ofdifferent operating wavelengths.

FIG. 3 illustrates how the exemplary unit cell construction shown inFIG. 2 may be designed to encode a range of different phase delays. Theunit cells shown in FIG. 3 are generally in the form of a rectangularcuboid with a square base shape of side λ₀/2 and height of λ₀. Thus, theunit cells allow the acoustic metamaterial layers to be quantised with aresolution of λ₀/2. This may be a good compromise betweenease-of-manufacture and the need to realise diffraction-limited fieldswithout spatial aliasing. Indeed, it has been found that it may beadvantageous to keep the size of the unit cells (in the plane of themetamaterial layers) smaller than the wavelength corresponding to theNyquist frequency. Thus, when designing a device that is optimised orconfigured for use at an operating wavelength, λ₀, the unit cells maysuitably have a dimension of λ₀/2, or smaller.

As shown in FIG. 3, and as mentioned above in relation to FIG. 2, theunit cells each comprise an open central channel having a structure thatdelays the incident wave, hence shifting the relative phase of theoutput. In particular, the open central channel is provided with alabyrinthine or meandered structure by a plurality of bars extendinginto the channel. The length of, b_(l), and spacing between, b_(s), themeanders may then be varied in order to provide a range of effectivepath lengths as shown in FIG. 3. In FIG. 3, the thickness of the wallsrelative to the configured operating wavelength, λ₀, is λ₀/40 and thethickness of the meanders is λ₀/20. However, these values may beselected as desired e.g. to achieve a desired strength or robustness, orbased on manufacturing constraints.

FIG. 4A shows a perspective view of an example of a pre-configured unitcell of the type shown in FIG. 3 that is pre-configured to encode aphase delay of 5π/8 for an operating wavelength λ₀. FIG. 4B shows incross-section 16 different unit cells that are pre-configured tointroduce phase delays spanning the range 0 to 2π in discrete steps ofπ/8. It can be seen from FIG. 4B how varying the lengths and spacing ofthe bars allows the phase delay to be adjusted.

The 16 different unit cells shown in FIG. 4B represent a set of 16unique quanta. The illustrated set of unit cells are uniformly spaced inphase and FIG. 4B thus represents a uniform 4-bit control (i.e. 16=2⁴).It has been found that any focussed field can be reproduced with anerror of less than 0.1 dB using such uniform 4-bit control. Using fewerquanta, or lower bit control, generally increases the error. Forinstance, the error may increase to about 1 dB for a uniform 3-bitcontrol (8 quanta), or about 3 dB with uniform 2-bit control (4 quanta).The error may be determined by comparing the analogue field that isdesired to be reproduced with the field generated by the spatial soundmodulation device.

Although the example set of unit cells shown in FIG. 4B are uniformlyspaced in the phase domain (in discrete intervals of π/8) it will beappreciated that a set of unit cells need not be uniformly spaced, andin embodiments, the set of unit cells may advantageously benon-uniformly spaced in the phase domain. For instance, by selecting anappropriate non-uniform set of quanta (i.e. phase delay values),practically any focussed field may be reproduced with similar error tothe uniform 4-bit control mentioned above but using fewer quanta. Forinstance, it has been found that a non-uniform 3-bit control may providesimilar results to a uniform 4-bit control. The optimum number and typeof unit cells for reproducing a given field with a desired accuracy maybe identified using digitisation techniques such as the wavelettransformation technique described below with reference to FIG. 12.

Furthermore, by stacking arrays or layers of unit cells togetherappropriately such that the phase delays combine additively, it may bepossible to realise each layer with a lower bit rate, so that a smallertotal number of unique unit cells is required. This advantage isparticularly evident when the phase quantisation is non-uniform. Thus,whilst any diffraction limited acoustic wave can in principle be createdusing a single acoustic metamaterial layer, stacking multiple acousticmetamaterial layers facilitates the generation of arbitrary acousticwaves at high accuracy and/or using fewer types of unit cell.

As best shown in FIG. 3, the base portions of the bars defining themeanders may have ‘shoulders’ such that they gradually taper into thechannel to the desired end thickness (e.g. λ₀/20). These ‘shoulders’ mayhelp to increase robustness and stability during manufacture and/or mayhelp contribute to impedance matching. In particular, the geometry ofthe unit cells may be selected so that the effective acoustic impedanceof each unit cell is matched to that of the ambient medium within whichthe device is operating (e.g. air or water), thereby increasing theefficiency of transmission (and suppressing reflection).

The unit cells illustrated in FIGS. 4A and 4B were designed foroperating at an operating wavelength of λ₀=8.66 mm (wavelength in air at25° C.), i.e. an operating frequency of 40 kHz within the ultrasonicrange. That is, the unit cell structures shown in FIG. 4B introduce thespecified phase delays spanning 0 to 2π to incident acoustic wavesprovided at the operating frequency of 40 kHz. Operating within theultrasonic range at 40 kHz may be particularly suitable for variousapplications using ultrasound, or ultrasonic carrier waves, includingsound from ultrasound applications. Most current studies of acousticmetamaterials only explore the audible range around 20 kHz or below.However, it will be appreciated that the techniques described herein maybe applied across a wide range of frequencies and the unit cells may inprinciple be designed for use with various suitable operatingwavelengths. It will also be appreciated that the set of unit cellsshown in FIG. 4B, even though designed for use with an operatingfrequency of 40 kHz, may still be used at other frequencies, but willthen work in a different way, as explained below with reference to FIG.6.

FIG. 5 shows a suitable frame within which the pre-configured unit cellsmay be inserted in order to form an acoustic metamaterial layer. Byre-arranging the unit cells within the frame, the spatial phasedistribution across the acoustic metamaterial layer may be re-configuredas desired. The frame may for instance comprise a laser-cut gridstructure. In the illustrated example, which is again configured for usewith an operating wavelength of λ₀=8.66 mm, although may be suitablyused or adapted for use with other operating wavelengths, the walls ofthe grid are about 1 mm thick. Each of the squares within the grid mayreceive up to four individual unit cells in a 2×2 assembly. However, itwill be appreciated that the frame may take various suitable forms. Forexample, in embodiments, each of the positions within the frame may bearranged to receive a single unit cell.

FIG. 6 shows the reflection and transmission properties for two of theunit cells shown in FIG. 4B. Particularly, FIG. 6 shows the amplitudeand phase frequency responses for unit cells that are pre-configured tointroduce phase delays of π (upper panel) and 15π/8 (lower panel).

As shown, each of the unit cells has a transmission co-efficient of (orvery close to) unity at the target operating wavelength of 40 kHz, andessentially zero reflectance, as expected. It will be appreciated thatthe unit cells do not therefore introduce unwanted energy losses, suchthat virtually all of the power from the acoustic source may betransmitted through the unit cell. This is particularly important wheremultiple unit cells or layers of unit cells are stacked together.Because of the very high (practically 100%) transmission of the unitcells, even when multiple layers of unit cells are stacked together, thestacked device may still transmit essentially all of the acoustic power.Furthermore, it has been found by measuring the phase response that theunit cells provide the desired phase delays for transmitted waves at 40kHz.

Although the example unit cells shown in FIG. 6 are designed foroperating at 40 kHz, it can be seen from FIG. 6 that the unit cellsdescribed above also transmit power at other frequencies, andparticularly at lower frequencies. It will be appreciated that althoughthe unit cell may transmit power across a range of frequencies, thephase response at different frequencies is not necessarily the same.Thus, although the device may be optimised for use at 40 kHz, the devicemay still be used at other frequencies, albeit potentially with someloss in transmission or with a change to the introduced phase value.However, so long as the actual operating frequency is known, it ispossible to determine the phase delay that will be introduced andarrange the unit cells appropriately to generate a desired output at theselected frequency.

Furthermore, it can be seen from FIG. 6 that there are a number of peakswherein the unit cells also have near total transmission. Indeed, sincethe additional phase delay depends on the product of the effectivelength, L_(eff), and the wavenumber of the incident wave, k, and thephase is restricted to the interval [0,2π ], there are a set offrequencies for which the unit cells have exactly the same transmissionperformance. In particular, the unit cells will have the sametransmission performance at all frequencies, f_(j)=f₀−jc₀/L_(eff),wherein j=0, 1, 2 . . . is an integer, f₀ is the operating frequency andc₀ is the speed of sound.

This multi-frequency response of the unit cells may be exploited forcertain applications, or in new types of acoustic devices, where it isdesired to generate more complex acoustic waves containing multiplefrequency components. For example, the multi-frequency transmission maybe exploited to allow multiple carrier waves at different ultrasonicfrequencies to be directed towards different places. Thismulti-frequency response may be facilitated by the symmetric structureof the unit cells. In other embodiments, asymmetric features or otherstructures may be provided within the unit cells in order to reduce themulti-frequency response. For example, the unit cells may be structuredto act as a filter so that substantially only power at the operatingwavelength is transmitted, where that is desired.

It is also contemplated that different unit cells within a particularsound modulation device or acoustic metamaterial layer may be configuredto operate at different frequencies so as to provide a broadband soundmodulation device. Where the sound modulation device is capable ofoperating over a range of frequencies, the incident acoustic wave maystill be mono-frequency, but the frequency of the acoustic source may bevaried in use without having to change the unit cells. It is alsocontemplated that the sound modulation device may be capable ofsimultaneously handling a range of different frequencies. One example ofa device that is capable of handling multiple frequencies is shown inFIG. 20, described below.

It is emphasised again that FIGS. 2 to 6 merely illustrate one exampleof a suitable unit cell for introducing a time delay, and that the unitcells may generally take various forms including, but not limited to,other types of labyrinthine or meandered structures, multi-slit, helicalor coiled structures, or Helmholtz resonator-type structures.

In a second main embodiment, the unit cells themselves are eachre-configurable between a plurality of different time delay values.Thus, in embodiments, the unit cells may be fixed in position within thearray or layer, but are re-configured in situ to encode a plurality ofdifferent phase delays. This is by contrast to the first main embodimentwhere the unit cells are fixed in phase, but may be re-positioned withinthe array. Thus, in the second main embodiment, the sound modulationdevice 20 may comprise one or more metamaterial layers 21,22 eachcomprising a two-dimensional array of re-configurable unit cells.

Naturally, it is also possible that in a given sound modulation deviceor metamaterial layer some of the unit cells may be both removable andre-configurable, or that some of the unit cells may be fixed in bothposition and phase. Furthermore, in embodiments, it is contemplated thata single sound modulation device or metamaterial layer may contain amixture of unit cells according to the first and/or second mainembodiments described above, and various combinations of unit cells arepossible.

The general form of the re-configurable unit cells according to thesecond main embodiment may be similar to those described above. That is,the unit cells may have a generally labyrinthine or meandered structure,e.g. as shown in FIGS. 2 and 3, or indeed any other suitable structurefor introducing an additional effective path length. However, ratherthan the unit cells having a fixed geometry, the re-configurable unitcells may be provided with one or more moveable or deformable elements.The moveable or deformable elements may be controllably moved ordeformed in order to vary the shape of the channel extending through theunit cells, and hence to vary the effective path length and phase delayintroduced by the unit cell.

In general, a re-configurable unit cell may be re-configurable betweenany number of states. Typically, however, the re-configurable unit cellsare re-configurable between a finite set of discrete phase delay values.For example, the unit cell may contain a plurality of flaps, with eachflap being independently controllable, such that the unit cell may be(re-)configured between a range of discrete states or phase values (i.e.for n flaps, there are 2^(n) possible states). That is, each of theflaps effectively provides a control bit. Where the unit cells within ametamaterial layer are re-configurable between a plurality of discretestates, the phase distribution across the sound modulation device ormetamaterial layer is again quantised both in the spatial and phasedomains. In this case, the quanta in the phase domain are defined by theavailable states of the plurality re-configurable unit cells.

In some embodiments the unit cells (or at least some of the unit cells)may be re-configured between only two states. That is, the unit cellsmay be re-configured between first and second states. For example, theunit cell may have only a single moveable or deformable element, or aplurality of moveable or deformable elements that are moved together.

FIG. 7A illustrates one example of a re-configurable unit cell accordingto the second main embodiment. As shown, the re-configurable unit cellcomprises one or more flaps 701,702 that are moveable between ON and OFFpositions such that when the flaps are ON, the flaps extend into thechannel to create a meander or substantially labyrinthine structure thatincreases the effective path length for acoustic waves passing throughthe unit cell, and when the flaps are OFF, the acoustic waves experiencea shorter effective path length or may pass straight through the unitcell. Thus, the unit cell may be controllable to change the position ofthe flap(s) 701,702, and hence change the effective path length (and theassociated phase delay) for acoustic waves passing through the unit cellbetween a plurality of discrete states. FIG. 7B illustrates how thephase delay introduced to an acoustic wave normally incident on the unitcell of FIG. 7A may be varied by changing the positions of the flap(s)701,702. Particularly, FIG. 7B shows the introduced phase delays whenthe flaps 701,702 are both OFF (left side panel), when one flap 701 isOFF whilst the other flap 702 is ON (center panel), and when both flaps701,702 are ON (right side panel).

Advantageously, an electronic controller or control circuitry may beprovided for controlling the re-configurable unit cells. The electroniccontroller or control circuitry may comprise or be connected to aprocessor and/or a storage device. The processor may generate therequired control signals to re-configure the unit cells in order toprovide a desired spatial phase distribution. Similarly, the storagedevice may store a number of profiles corresponding to variouspre-determined spatial phase distributions which are then passed to theprocessor or electronic controller to generate the required controlsignals. The electronic controller or control circuitry may take intoaccount the frequency of operation in order to determine which unitcells should be arranged where in the array in order to generate thedesired spatial phase distribution. For instance, given a desiredoperating frequency, the control software may determine the requiredtime delays and unit cells for generating the desired spatial phasedistribution. In other embodiments, the control software may determineor control the operating frequency in order to generate the desiredspatial phase distribution.

It will be appreciated that the arrayed or layered structure of thesound modulation devices described herein lends itself to incorporationwithin existing stacked geometries known e.g. for LED or transistordevices, such that similar manufacturing techniques and control systemsmay be used. For instance, FIG. 8 shows a stacked device comprising twoacoustic metamaterial layers 801,802 alternately arranged between a pairof thin film transistors defining electrodes 803,804 for providingcontrol signals for controlling the unit cells within the acousticmetamaterial layers.

An electronically re-configurable unit cell e.g. of the type shown inFIG. 7A may be manufactured by coating the interior of the unit cells,or particularly the moveable or deformable elements thereof, with adielectric material or a charged powder.

Alternatively, the moveable or deformable elements may be formed from adielectric or piezo-electric material. The unit cells may then beconnected to electrodes for providing control signals for switching theunit cell between states. Another example of a possible technique formanufacturing a re-configurable unit cell employing a layeredconstruction will be described below with reference to FIG. 14 and FIGS.15A & 15B.

The unit cells within a metamaterial layer may in embodiments each bere-configured independently. In other embodiments the electroniccontroller or control circuitry may be arranged to control groups orsub-groups of unit cells together. This may help to simplify theelectronic control requirements and particularly the arrangement of theelectrodes and/or the complexity of the control signals. Alternativelystill, actuation may come from a pneumatic or a microfluidic system.

It will be appreciated that by using a computer or other processor tocontrol a set of re-configurable unit cells allows the acoustic wave tobe re-configured essentially in real-time. Furthermore, because thespatial sound modulation device 20 may be independent of the acousticsource 10, the spatial sound modulation device 20 may be re-configuredto generate a different acoustic wave without any loss of power. Thisprovides a significant advantage over current phased transducer arraytechnologies.

Although the discussion above has referred to unit cells each capable ofencoding a particular phase delay or phase delays, in some embodiments,a plurality of unit cells may be fixed together to form a single block(or “sub-array”) of unit cells. For example, a block may comprise a 2×2,or 3×3, array of unit cells, or generally an m×n array of unit cells.Furthermore, a block may comprise a stack of unit cells or arrays ofunit cells. For instance, a block may comprise two or three arraysstacked together to form a three-dimensional block (e.g. a cubic arrayof 3×3×3 unit cells). The block(s) may be configured to perform acertain specific function or transformation. That is, the unit cellswithin the block may be selected in order to provide a particular phasedistribution. Thus, instead of re-configuring or removing unit cellsindividually, a block of cells may be re-configured or removed together.While it may be desired for the spatial resolutions of the unit cellswithin a block to remain at λ₀/2 or lower to reduce higher orderdiffraction effects, the use of blocks comprising a plurality of unitcells may facilitate the mechanical assembly and/or electronic controlof the devices, helping to make the sound modulation device morecost-effective.

Common to both of the main embodiments described above is the concept ofre-configuring the sound modulation device or the acoustic metamateriallayers in order to alter the spatial phase distribution. By appropriateselection of the phase delays at each position within the arrays orlayers, it is possible to perform a variety of acoustic manipulationsand to generate essentially arbitrarily complex (diffraction limited)acoustic waves.

For example, the spatial sound modulation device, or a layer thereof,may be arranged to perform a focussing transformation. That is, thedevice may be configured to focus an incident acoustic wave towards afocal point. The basic focussing transformation may be described by theanalogue phase distribution

${{\phi \left( {x,y} \right)} = {\phi_{0} - {\frac{2\pi}{\lambda_{0}}\left( {\sqrt{r^{2} + F_{0}^{2}} - F_{0}} \right)}}},{{{where}\mspace{14mu} r^{2}} = {x^{2} + {y^{2}.}}}$

FIG. 9 illustrates an example of an acoustic metamaterial layerconfigured to provide a focussing transformation at 40 kHz. Themetamaterial layer shown in FIG. 9 is formed of 16 different phasevalues, e.g. corresponding to the 16 phase values between 0 and 2π insteps of π/8 shown in FIG. 4B. However, it will be appreciated that thelayer may equally use alternative arrangements of unit cells, that maybe either pre-configured or re-configurable, and may be either uniformlyor non-uniformly spaced in the phase domain. In whatever form they take,the unit cells or blocks of unit cells at each position (i,j), within asingle metamaterial layer are selected or configured to have a phasevalue that most closely matches the desired phase as defined by theanalogue phase distribution φ(x,y) above. For FIG. 9, that means theunit cells at each position are selected to have a phase value selectedfrom the 16 available phase values that most closely matches the desiredphase. The acoustic metamaterial layer thus contains a quantisedrepresentation φ_(i,j) of the analogue phase distribution φ(x,y). Toaccount for the presence of the frame, etc. the phases assigned to theunit cells may be taken as the phase according to the analogue phasedistribution φ(x,y) corresponding to an imaginary point at the centre ofeach unit cell.

FIG. 9 also shows pressure plots illustrating the acoustic wave in thevertical plane moving away from the surface of the metamaterial layerand in the horizontal plane at a position 100 mm from the surface. Itcan be seen that the spatial sound modulation device performs asexpected by focussing the acoustic wave.

It has been found that the size of the focal region perpendicular to theaxis depends on the lateral dimensions of the acoustic metamateriallayer. In particular, the larger the lateral dimensions of the acousticmetamaterial layer, the tighter the focus.

As another example, the sound modulation device, or another layerthereof, may be arranged to perform a steering transformation. That is,the sound modulation device, or layer, may be arranged to steer orre-direct the incident acoustic waves to a different position away fromthe central axis of the device.

In embodiments, various transformations may be combined. For example,the focussing and steering transformations described above may becombined in order to perform a “steered focussing”. As another example,a focussing transformation may be combined with a half-wavetransformation in order to create a trap for acoustically levitatingobjects.

Stacking two metamaterial layers together may allow steered focussingover larger angles away from the axis than may typically be possiblewith a single metamaterial layer, for instance, steered focussingoutside the lateral boundaries of the sound modulation device. This isillustrated in FIG. 10 which shows how a focussing layer (like thatshown in FIG. 9) may be stacked underneath a steering layer. FIG. 10also shows the simulated and measured acoustic fields in the verticalplane moving away from the surface of the metamaterial layer.

Thus, stacking multiple acoustic metamaterial layers togetherfacilitates performing relatively complex manipulations of acousticwaves. Each metamaterial layer in the stack may generally be arranged toperform a certain transformation so that the stacking adds thetransformations together. This concept is illustrated in FIG. 11 for the‘bottle beam’ acoustic field shown in FIG. 1. This bottle beam may beused as a tractor beam or for acoustic levitation, where the innerdiameter of the bottom annulus of the bottle controls the tightness ofthe acoustic trap. As shown in FIG. 11, the bottle beam field may begenerated by stacking a first acoustic metamaterial layer that providesa bottle signature with a focussing acoustic metamaterial layer.

Also presented herein are techniques for, given a desired acousticfield, determining how the phase delays (e.g. unit cells) within theacoustic metamaterial layer(s) or the sound modulation device should bearranged in order to recreate the desired acoustic field.

It will be appreciated that real-life acoustic fields have a continuousspatial phase distribution, whereas the spatial phase distribution ofthe acoustic metamaterial layers according to the invention isquantised. According to the preferred embodiments described herein, thespatial phase distribution is quantised in both the spatial and phasedomains. Thus, what is required is essentially a process ofanalogue-to-digital conversion (or “digitisation”) with two parameters:one in the spatial domain, which depends on the size of the unit cellsand of the number of unit cells within the metamaterial layer or array;and one in the phase domain, which governs the number of availableunique phases provided by the unit cells.

Generally, a method for optimising the configuration or design of ametamaterial layer or stack of metamaterial layers to generate a givenacoustic field may involve sampling and quantising the acoustic field toproduce a quantised representation of the spatial phase distribution.The quantised representation may then be used to determine which phasedelays should be used at which positions in order to recreate theoriginal acoustic field. This method may be implemented via software.This process is generally illustrated in FIG. 12.

In particular, FIG. 12 shows the process of digitisation for an analogueacoustic field 500 having a central focus.

The process starts with a first step of sampling the spatial phasedistribution of the analogue acoustic field 500. The acoustic field 500may be sampled at a certain distance from the plane of the soundmodulation device, and acoustic holography techniques may then be usedto obtain the sampled acoustic field phase distribution 501 in the planeof the sound modulation device.

The sampled acoustic field 501 is then quantised in the spatial andphase domains to generate a quantised spatial phase distribution 502.The phase delay and position values from the quantised spatial phasedistribution 502 may then be mapped onto appropriate unit cells. Thus,it is possible to determine which unit cells (i.e. having which phasedelays) should be used at which positions in order to reproduce theoriginal acoustic field 500 with the desired accuracy.

In some cases, the quantised spatial phase distribution 502 may have adifferent resolution in the spatial and/or phase domain than that whichis achievable based on the available unit cells. Thus, the process mayinclude a final step in which the quantised spatial phase distribution502 is compressed in the spatial and/or phase domains in order to mapthe quantised spatial phase distribution 502 to the available unitcells.

The result of this compression step is a compressed spatial phasedistribution which may be directly mapped to the available unit cells.That is, the compressed spatial phase distribution may contain quantahaving the same physical dimensions and range of phase delays as theavailable unit cells.

The digitisation technique illustrated in FIG. 12 is essentially basedon the recognition that the phase distribution of the acoustic field maybe treated as a two-dimensional ‘image’, except with the imagerepresenting the phase delay at each spatial position rather than e.g. acolour. Furthermore, because the techniques described herein involve aquantisation of the spatial phase distribution according to the size andphase(s) of the unit cells, mapping the phase ‘image’ onto ametamaterial layer essentially involves steps of digitisation and imagecompression, and various techniques for encoding and compressing thespatial phase distribution may be used analogously to conventional imagecompression techniques.

For example, one widely known compression algorithm is the JPEG standardof image encoding which is based on discrete cosine transforms. However,JPEG compression may not be particularly suitable for compressingacoustic spatial phase distributions as it cannot accurately captureabrupt changes in phase.

Preferably, therefore, a generic feature preserving compressiontechnique is used that can accurately capture edges or abrupt changes inphases within the spatial phase distribution. In embodiments, thecompression algorithm uses wavelet transformations such as the discretewavelet transform. Wavelet transformations are specifically aimed atdetermining the lowest number of coefficients necessary for a specifiedreconstruction quality of localised features. Thus, wavelettransformations may be particularly suited for optimising or determiningthe optimum number of unit cells or re-configurable phases needed torecreate a given acoustic field.

The basis functions for the wavelet transformation (or “wavelets”)generally have an average value of zero, and the image i.e. the spatialphase distribution may be decomposed into a superposition of shifted andscaled representations of the original mutually orthogonal wavelets. Inembodiments, the Haar wavelet is used as the basis function for thewavelet transformation. The Haar wavelet, as shown in FIG. 13, is asquare wave over the λ₀-wide interval of definition. The Haar waveletmay thus be mapped onto unit cells. For instance, two adjacent unitcells having opposite phases may be described by a Haar function. Thatis, the unit cells may contain the same phase and spatial information asthe Haar wavelet.

Because the Haar wavelet itself is discontinuous, it may be particularlysuitable for handling ‘images’ with sharp edges such as the spatialphase distributions of an acoustic field. However, other suitablewavelets may also be used. For example, another suitable wavelet may bethe Shannon wavelet i.e. the function, S(x)=2(sin 2πx−cos πx)/(π−2πx),over the λ₀-wide interval of definition.

The discrete wavelet transform represents an image over a plurality ofdifferent scales, selecting at each step (i.e. scale) the key features,with low spatial frequencies, and the residual features, with highspatial frequencies. In this way, a hierarchical tree of matrices isgenerated where the spatial resolution doubles at each step. That is, asillustrated in FIG. 13, each ‘pixel’ in the original 16×16 pixel imageis decomposed into a series of four matrices ({0}, {1}, {2}, {3}) usinga Haar function defined over an interval of one wavelength, λ₀. Each ofthese matrices is then further decomposed into a series of furthermatrices ({0,0}, {0,1}, {0,2}, {0,3}) using a Haar function over aninterval of two wavelengths, 2λ₀, and so on, down to a desired level K.In FIG. 13 the matrix stops after three decompositions. Thus, the finallevel of the hierarchy in FIG. 13 is a set of 2×2 matrices.

Once the tree of matrices is obtained, the compression procedure worksby computing wavelet decomposed spatial phase distribution φ^(DWT) _(K)up to a level K, with the coefficients below a certain threshold value δset to zero, and finally computing the inverse transform. The inversetransform contains a number of unique phases dependent on the thresholdvalues chosen. The threshold values thus determine the compression levelfor the image. As information is reduced by the thresholding step, thisis a lossy process, and the inverse transform in general contains lessinformation and a smaller number of required phases than the originalquantised spatial phase distribution 502.

The phases are generally are not uniformly distributed either spatiallyor in the phase domain. Thus, given a particular set of unit cells, i.e.a particular set of available phase delay values, it is necessary tomatch the closest possible unit cell to the phase delay value given bythe inverse transform. Alternatively, once the inverse transform iscomputed, and the required phases identified, the unit cells may bepre-configured to match the required (typically non-uniformlydistributed) phases. The accuracy at which the field can be reproducednaturally thus depends primarily on the compression step i.e. to whatextent the available unit cells are capable of accurately matching thequantised spatial phase distribution 502.

FIG. 13 depicts a 4-bit encoding of the image (i.e. using 16 uniquequanta). In general, the number of unique quanta needed to realise theinverse transform decreases with increasing values of δ. For example, 8unique quanta (3 bits) are sufficient for a compression rate at 4:1,whereas only 6 unique quanta are needed for a compression rate at 4.6:1,and only 4 unique quanta (2 bits) may be sufficient for a compressionrate at 8:1. Suitable threshold values may be determined throughoptimisation based on the error in approximating the continuous phasedistribution.

It has been found that a uniform 4-bit quantisation is sufficient torealise practically any arbitrarily complex acoustic field with anaccuracy of 0.1 dB. 4-bit quantisation may e.g. be achieved either usinga single acoustic metamaterial layer with 2⁴=16 unique phase delays/unitcells, or by stacking two acoustic metamaterial layers each having 8unique phase delays/unit cells. At higher bit rates, the differencebetween the resulting field and the one obtained with 4-bit quantisationis typically too small to be significant, depending on the application.It has been found that a non-uniform 3-bit quantisation may also besufficient for realising practically any arbitrarily complex acousticfield with an accuracy of 0.1 dB. In some applications, uniform 3-bit or2-bit quantisation may be sufficient depending on the requiredprecision.

As shown in FIG. 12, the phase distribution may be compressed using thewavelet transformation onto a single layer of unit cells 503. Thiscompression is inherently lossy. However, a theoretically losslesscompression is also possible. For instance, using the Haar function as aparent wavelet, at the first level of the wavelet hierarchy this is asignal of amplitude 1 over the range 0 to λ₀/2 and of amplitude−1 overthe range λ₀/2 to λ₀. As illustrated in FIG. 13, the original 4-bitimage may be decomposed into a quantised spatial phase distribution:φ_(i,j)={0}×H_(λ0)+{1}×H_(λ0)+{2}×H_(λ0)+{3}× H_(λ0), where {0}, {1},{2} and {3} are the matrices shown in FIG. 13 and H_(λ0) is the spatialrepresentation of the Haar function. However, since phase delays areadditive, this sum can be performed physically by stacking fourdifferent acoustic metamaterial layers, with each acoustic metamateriallayer representing one of the matrices {0}, {1}, {2} and {3}corresponding to the first level of the wavelet hierarchy.

FIG. 12 also illustrates the theoretically lossless (or at least lowloss) compression technique, where the compression results at firstlevel in a three branch tree representing the structure of thedecomposition. The quantised spatial phase distribution 502 may thus bematched to a stack of three acoustic metamaterial layers 504.

For the scheme shown in FIG. 13, each coefficient of the wavelettransform then applies to a 2×2 array. In terms of unit cells, thismeans that in each 2×2 array, the first two unit cells should have aphase given by the corresponding coefficient of the wavelet transformand the other two unit cells will have an opposite phase. These 2×2arrays may comprise the blocks of unit cells described above, or maycomprise independent unit cells.

The output of the digitisation process is thus a phase map showing whichunit cells i.e. which phase delays should be introduced at whichpositions in order to best reproduce the original acoustic field. Thus,for pre-configured unit cells, the digitisation process essentiallyprovides a parts list (i.e. the number and type of unit cells required)and assembly instructions for constructing the acoustic metamateriallayer or sound modulation device. Where the unit cells arere-configurable, the output may be provided to the electronic controllerand used to define appropriate electronic control signals forre-configuring the unit cells.

Although FIGS. 12 & 13 illustrate an example wherein quantisation isperformed in the spatial and phase delay domains (only) it will beappreciated that quantisation may also (or alternatively) be performedin the spatial and amplitude/intensity domains. For instance, it hasbeen found that the frequency response of the unit cells may vary withtransmission. The transmission efficiency of the unit cells may thusalso be used to adjust the acoustic output (e.g. frequency response)device. Thus, instead of (or as well as) quantising the phasedistribution in the phase delay domain, a step of quantising the phasedistribution in the amplitude (or intensity) domain may also beperformed.

In general, the unit cells described herein may suitably be formed usingvarious microfabrication techniques. The channel topology may thus bedesigned to be suitable for microfabrication. In some examples, the unitcells may suitably be manufactured from a thermoplastic material. Forinstance, where one or more bars are provided as shown in FIG. 3, thebars may be extruded or pulled out from the walls of the unit cells. Insome embodiments, the unit cells may suitably be manufactured by 3Dprinting.

However, a preferred manufacturing approach, at least for someapplications, is to utilise a layered construction, wherein athree-dimensional unit cell is constructed by stacking together aplurality of relatively thin two-dimensional layers.

FIG. 14 illustrates an example of this preferred unit cell constructionwherein a unit cell 140 is constructed from a plurality of layers 142that are stacked together. As shown, in FIG. 14, the layers extendparallel to the surface of the unit cells within the device (in the x-yplane as shown). Each of the layers comprises an opening and the layersare arranged so that the respective openings are aligned and overlappingto define an open central channel 144 in the form of a folded physicalpath extending through the unit cell 140 (in the z-direction). Thus, theunit cell 140 appears geometrically like a rectangular cuboid with asquare base-shape.

The internal structure of the unit cell 140 includes a central openchannel 144 that acts to delay an incident acoustic wave (henceintroducing a relative time or phase delay). The open channel 144 thusincludes a folded physical path along which the acoustic wavespropagate. The open channel 144 may generally be filled with either thesurrounding fluid (e.g. air or water depending on the application), or adifferent fluid, if desired, e.g. to further modify the properties ofthe incident acoustic wave. However, in both cases, the time delay maybe (primarily) determined by the structure of the unit cell 140 and theproperties of the fluid within the open channel 144 do not significantlychange during use.

The layers may be fabricated using various suitable MEMS techniques. Forinstance, a pattern may be determined for each layer indicating therequired shape and position of the opening, and the layers may then beconstructed individually by etching or otherwise removing material inthis area. MEMS fabrication techniques are in general very welldeveloped and are capable of defining very high resolution features,e.g. at (sub) micron level. For instance, the layers may be pattered bysuitably etching away material to define the openings. This may beperformed using high precision laser cutting, chemical etching orphotolithography. The use of a layered construction may thus provide ahighly scalable approach for fabricating large numbers of unit cellswith high precision and resolution suitable for operation over a widerrange of frequencies than might be possible e.g. with current generation3D printing techniques. Furthermore, MEMS techniques are typicallycurrently much faster and cheaper than traditional additivemanufacturing techniques such as 3D printing.

FIGS. 15A and 15B illustrate how by changing the shape and position ofthe opening(s) in the layers it is possible to generate different unitcell structures encoding different time delays. For instance, by varyinge.g. the width of the channel and the width of the opening of thechannel, the resulting phase may be shifted to cover a 2π span. This maybe done during the manufacture, i.e. to fabricate multiple differentpre-configured unit cells each encoding a specific time delay. However,it is also contemplated in embodiments that the layers within a unitcell may be slid relative to one another in use (e.g. under electroniccontrol) in order to provide a re-configurable unit cell. That is,because the unit cell is formed from a plurality of individual layers,it is possible to shift the individual layers relative to one another inorder to dynamically adjust the structure of the unit cell to vary thetime (or phase) delay that is introduced. This adjustment may also beused to increase the frequency band where each unit cell can operate.For instance, by introducing small vibrations in the layers that composethe unit cell, and thereby causing small variations in the length of thechannel, it is possible (with appropriate unit cell designs) to maintainthe transmission efficiency of the unit cell over a wider range offrequencies without significantly impacting the output phase of the unitcell. In order to avoid artefacts, the layers may be physically vibratedat frequencies at least twice as high as the acoustic operatingfrequency of the device (i.e. based on the Nyquist criterion). However,it has been found that the fidelity of the acoustic output may still beimproved with lower vibration frequencies than this. (This effect isgenerally analogous to creating displaced images in modern 3Dtelevisions.)

The unit cell shown in FIG. 14 has an open channel 144 that is foldedessentially in only two dimensions (i.e. in the x-z plane as shown). Thechannel thus extends continuously through the cell (i.e. in thez-direction). However, it is also possible using the layeredconstruction to create unit cells having more complex three dimensionalgeometries. FIG. 16 shows an example of an open channel that is foldedin three dimensions that may suitably be constructed by providingsuitably patterned layers. As shown, the open channel in FIG. 16 is nowalso folded in the third (y) direction to create a fullythree-dimensional meandered structure. This may provide variousadvantages both from an acoustic and manufacturability perspective. Forinstance, the number of layers in the stack required to produce acertain time delay may be reduced, allowing the thickness of the unitcell to be reduced further, e.g. to (ultra) sub wavelength thicknesses.

In general, the layers may extend either in the plane of the device(i.e. in the x-y plane as shown in FIG. 14), or perpendicularly thereto(in the x-z or y-z planes). For instance, it can be seen that thethree-dimensional channel shown in FIG. 16 may be constructed by slicingthe unit cell in any of these planes, and then fabricating suitablelayers that can be stacked together to define the three-dimensional unitcell.

FIG. 17 shows another example of a unit cell construction wherein theunit cell 170 comprises a plurality of layers 172 that are stackedperpendicularly to the plane of the unit cells within the device (i.e.the x-z plane as shown). In this case, the channel structure 174 ispatterned directly onto each of the layers 172, with the layers thenbeing stacked together to extrude the pattern in the third (y)direction. Although illustrated in FIG. 17 as a relatively simplerectangular pattern, it will be appreciated that this approach may allowfor more complex channel structures to be defined, e.g. including curvedor rounded portions. By contrast, the parallel stacking shown in FIG. 14is fundamentally limited to substantially rectangular features which aredefined by the thicknesses of the layers (although it would e.g. bepossible to introduce a taper to some of the openings).

FIGS. 14 to 17 all show examples of single unit cells that may beconstructed from a plurality of layers stacked together. However, thelayered construction may also advantageously be used to construct asingle acoustic meta-surface comprising a plurality of unit cells. FIG.18 shows an example of this. In particular, FIG. 18 shows a set ofeleven layers 182 that may be stacked together to define an acousticmeta-surface. As shown, each layer comprises a pattern representing aportion or slice of the unit cell at that position within the array. Thelayers 182 can thus be stacked together to assemble a meta-surfacecomprising an array of unit cells.

It will be appreciated that constructing a single acoustic meta-surfacefrom a plurality of layers may provide even further advantages from amanufacturing perspective as now it is only necessary to fabricate oneset of layers for the entire device (surface) rather than multiple setsfor each unit cell. For instance, a suitable lithography mask can bedesigned for each of the layers 182, and these can then each bemanufactured on demand in a single step before being stacked together.

The layered construction also offers a large amount of flexibility onthe structure of the unit cells. For instance, by using a layeredconstruction, e.g. as shown in FIG. 18, it is possible to essentiallyarbitrarily set the time (or phase) delay values for each of the unitcells within the array. Thus, there may be a much enhanced ‘palette’ ofphase delay values that may be used for reconstructing an analogueacoustic field.

FIG. 19 illustrates an example of an improved non-uniform quantisationthat may be achieved in this way. For instance, as shown, the desiredacoustic field may generally be represented as an 8-bit phase map. Incases where the number of available unit cells is limited (e.g.) to aset of 8 uniformly spaced quanta, so that only a 3-bit quantisation ispossible, naturally some detail is lost from the analogue phase map. Formost applications, as described above, this is acceptable, as the erroris often low enough to not be readily perceptible (e.g. about 1 dB orless). However, for applications where higher precisions are desired, ahigher bit rate quantisation might be desired. The layered constructiondescribed above allows a larger number of unit cells having differentphase delays to readily be fabricated. Thus, instead of choosing auniform quantisation based on a finite number of available phase delayvalues, it is possible to use precisely the needed value (or at leastrounded to the nearest available) from a wider pool (or ‘palette’) ofphase delay values.

As described above, a unit cell may typically be configured for optimaloperation at a certain (single) frequency. However, in some cases, itmay be desired to provide a device that is capable of handling multipledifferent frequencies. FIG. 20 shows one example of a device capable ofdoing this. As shown, the device generally comprises a number ofdifferent types of unit cells that have different transmissioncharacteristic (see the inset graph). Particularly, in the example shownin FIG. 20 (although other arrangements are of course possible) thedevice comprises three different types of unit cells 200A, 200B, 200Cthat are respectively configured for optimal operation at frequenciesf₁, f₂, f₃. In FIG. 20 each of the first 200A, second 200B and third300C unit cells are configured to perform the same operation, but atdifferent frequencies. In order to provide a multi-frequency response, amoveable masking element 206 is provided that selectively covers thedifferent types of unit cells. For instance, as shown, in a firstposition, the masking element 206 may be used to cover the first 200Aand third 200C unit cells so that acoustic waves that have passedthrough the first and third unit cells are blocked, whilst leaving thesecond 200B unit cell exposed so that acoustic waves that have passedthrough the second unit cell 200B are transmitted past the maskingelement 206 and generate the acoustic output. The masking element 206may then be moved between different positions in which the first 200Aand third 200C unit cells are exposed (whereas the other unit cells arecovered) in order to switch the device between the operating frequenciesf₁, f₂, f₃.

It will be appreciated that various other suitable arrangementsutilising a masking element 206 may be possible. For instance, FIG. 21shows a similar arrangement but wherein the three types of unit cells202A, 202B, 202C are now configured to operate at the same frequency butto perform different operations at that frequency. Thus, as shown, byselectively covering different of the types of unit cells, so that onlyone type of unit cells are left open, the device may be rapidly switchedbetween different functionalities by simply moving the mask 206. Thus,as shown, by moving the mask between the different positions, whereinthe different types of unit cells are exposed, the device may beswitched between a first function wherein the acoustic field is focussedcentrally, a second function wherein the acoustic field is steered tothe left and a third function wherein the acoustic field is steered tothe right.

It will be appreciated that the techniques and devices for spatiallymodulating sound described herein may found application in a variety ofcontexts.

For example, in embodiments, the techniques described herein may be usedto realise a directional sound system such as an ‘audio spotlight’ usedwith digital signage, or displays, or kiosks for targeted advertising orannouncements. A directional sound system may also be employed onconsumer electronic devices e.g. for providing wireless audio devices.

Alternatively, the directional sound system may utilise the sound fromultrasound effect such that the acoustic output is effectively carriedby a modulating ultrasonic wave (e.g. at 40 kHz).

A device for use in these contexts may comprise a sleeve for mountingonto an existing speaker in order to provide focussing and/or steering.For instance, a device may be mounted onto an existing directional orfocussed speaker in order to provide additional steering.

As another example, the techniques may be used for wireless powertransfer such as ultrasonic charging. Existing techniques for wirelessultrasonic charging using a phased transducer array require extremelyhigh operating powers in order to provide a sufficiently strong focussedbeam, and may not therefore be practical or safe. As explained above,because the power requirements for the modulator are separated from thepower requirements of the source, the techniques described herein mayoperate at significantly lower powers than phased array techniques.

A further example would be using the acoustic wave for interactions withother objects, for instance, in the field of haptic control e.g. forconsumer electronic devices, or for acoustic levitation or tractorbeams. Similarly, the techniques may be used in virtual realityapplications.

The techniques may also find a variety of application in the medical andindustrial sectors. For instance, there are a variety of therapeutic anddiagnostic techniques involving spatial sound modulation. One example ofthis would be High Intensity Focussed Ultrasound for ablating tissue.Another example would be targeted drug delivery. A typical industrialapplication may be in the field of non-destructive testing, or for wastemanipulation.

Although FIG. 1 shows a transmitting device, it will be appreciated thatthe devices substantially as described herein may also be used as partof a receiver or sensor assembly, for example, for acoustic sensing orimaging applications. Typical acoustic sensing or imaging applicationsmay include medical imaging, or proximity sensing (e.g. in a motion orposition sensor). In this case, the device may be used to transform anincident acoustic wave (or field) into a form that is more suitable forsensing or imaging purposes.

Similarly, although FIG. 1 shows a device that operates in transmission,it will be appreciated that the devices substantially as describedherein may also be operated in reflection.

Also, although FIG. 1 shows a stand-alone acoustic manipulation deviceit will be appreciated that the devices (and unit cell structures)substantially as described herein may also be incorporated as part of alarger structure. In this way, the structure is provided with theability to spatially modulate acoustic fields. This may be advantageousfor instance for a variety of noise control applications. For example,such devices could be incorporated into building materials (such ashouse bricks or insulation). FIG. 22 shows an example of a structure 220having an acoustic manipulation device 222 incorporated into its surfacein order to provide noise control (e.g. noise reduction). The device 222may be provided as an external layer onto the surface of the structure220 or may be formed integrally with the structure (e.g. by formingsuitable unit cell structures in the structure, e.g. on the exteriorsurface thereof).

In general, the devices described herein may be used in any medium,depending on the application. For instance, the devices may be used inair, or may be used in a liquid medium e.g. in the context of medical orindustrial imaging.

Although the techniques presented herein have been described withreference to particular embodiments, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing from the scope of the invention as set forth in theaccompanying claims.

1. A device for manipulating an incident acoustic wave to generate anacoustic output comprising: a plurality of unit cells arranged into anarray, at least some of said unit cells being configured to introducetime delays to an incident acoustic wave at the respective positions ofthe unit cells within the array of unit cells, such that said pluralityof unit cells define an array of time delays to thereby define a spatialdelay distribution for manipulating an incident acoustic wave togenerate an acoustic output, wherein the array of time delays defined bysaid plurality of unit cells is re-configurable to vary the spatialdelay distribution in order to generate different acoustic outputs. 2.The device of claim 1, wherein the time delay introduced by a unit cellis determined by the path length for acoustic waves travelling throughthat unit cell.
 3. The device of claim 1 or 2, wherein said plurality ofunit cells comprises a plurality of pre-configured unit cells, eachpre-configured unit cell introducing a fixed time delay to the incidentacoustic wave.
 4. The device of claim 3, wherein said array of timedelays may be re-configured by changing the type and/or position of atleast some of the pre-configured unit cells within the array of unitcells.
 5. The device of any preceding claim, further comprising a frameor mounting structure, and wherein said plurality of unit cells arereleasably mounted on or within said frame or mounting structure.
 6. Thedevice of any preceding claim, wherein the plurality of unit cells arereleasably connectable to each other in order to define said array. 7.The device of any preceding claim, wherein said array of unit cellscomprises a plurality of blocks of unit cells, each block comprising afixed arrangement of unit cells.
 8. The device of claim 6, wherein eachblock of unit cells comprises a fixed arrangement of pre-configured unitcells arranged so as to provide a pre-determined manipulation of anincident acoustic wave, optionally wherein said pre-determinedmanipulation acts to: (i) focus said incident acoustic wave; (ii) steeror direct said incident acoustic wave; and/or (iii) introduce a phasedelay of π radians to said incident acoustic wave.
 9. A kit of parts forforming a device as claimed in any of claims 1 to 8, comprising aplurality of different types of unit cell or blocks of unit cells. 10.The kit of claim 9, further comprising a frame or mounting structure formounting said plurality of different types of unit cell or blocks ofunit cells in an array.
 11. The device of any preceding claim, whereinsaid plurality of unit cells comprise a plurality of re-configurableunit cells that may each be selectively re-configured to cause the unitcell to introduce different time delays.
 12. The device of claim 11,wherein each re-configurable unit cell may be selectively re-configuredbetween a set of two or more discrete time delay values.
 13. The deviceof claim 11 or 12, wherein each re-configurable unit cell comprises oneor more moveable elements moveable between a plurality of positions inorder to vary the time delay introduced by the re-configurable unitcell.
 14. The device of any of claims 11 to 13, wherein each of saidre-configurable unit cells may be electronically re-configured.
 15. Thedevice of any preceding claim, wherein each of the unit cells has anassociated amplitude value representing the relative amplitude, orchange in amplitude, introduced by that unit cell to an incidentacoustic wave, and wherein the array of unit cells may be re-configuredto vary an amplitude distribution of the array of unit cells in order togenerate different acoustic outputs.
 16. The device of any precedingclaim, wherein each of said unit cells is formed from a plurality oflayers that are stacked together such that the structure of the unitcells is defined by the plurality of layers in combination.
 17. A devicefor manipulating an incident acoustic wave to generate an acousticoutput comprising: a plurality of unit cells arranged into an array, atleast some of said unit cells being configured to introduce time delaysto an incident acoustic wave at the respective positions of the unitcells within the array of unit cells, such that said plurality of unitcells define an array of time delays to thereby define a spatial delaydistribution for manipulating an incident acoustic wave to generate anacoustic output, wherein the structures of the unit cells within thearray are defined by a plurality of layers in combination.
 18. Thedevice of claim 17, wherein each layer comprises a portion or slice ofeach of the plurality of unit cells, so that the array of unit cells isdefined by the plurality of layers stacked together.
 19. The device ofclaim 18, wherein the plurality of layers are stacked in a fixedarrangement to encode a fixed spatial delay distribution.
 20. The deviceof any of claims 16 to 18 wherein the unit cells may be re-configured tointroduce different time delays by moving or sliding the layers withineach unit cell relative to one another to change the structures of theunit cells.
 21. The device of any of claims 16 to 20, wherein one ormore openings are provided in each of said layers so that when thelayers are stacked together the openings form a channel extendingthrough the unit cell.
 22. The device of any of claims 16 to 21, whereinthe plurality of layers that form the unit cells are stacked parallel tothe plane and/or surface of the array; or wherein the plurality oflayers that form the unit cells are stacked perpendicularly to the planeand/or surface of the array.
 23. The device of any of claims 16 to 22,wherein the layers are caused to vibrate relative to each other in useto increase the frequency band where each unit cell can operate.
 24. Adevice for manipulating an incident acoustic wave to generate anacoustic output comprising: a plurality of unit cells arranged into anarray, at least some of said unit cells being configured to introducetime delays to an incident acoustic wave at the respective positions ofthe unit cells within the array of unit cells, such that said pluralityof unit cells define an array of time delays to thereby define a spatialdelay distribution for manipulating an incident acoustic wave togenerate an acoustic output, wherein the device is adjustable in orderto generate different acoustic outputs.
 25. The device of any precedingclaim, wherein said plurality of unit cells are configured to introducea desired phase delay to an incident acoustic wave at an operatingwavelength, λ₀, and wherein at least some of, or each of, said pluralityof unit cells has a dimension within said array of unit cells of halfsaid operating wavelength or smaller.
 26. The device of any precedingclaim, wherein the spatial delay distribution of said array isconfigured so as to focus an incident acoustic wave.
 27. The device ofany preceding claim, wherein the spatial delay distribution of saidarray is configured so as to steer or direct an incident acoustic wave.28. The device of any preceding claim, wherein the spatial delaydistribution of said array is configured so as to introduce a phasedelay of π radians to an incident acoustic wave.
 29. The device of anypreceding claim, wherein the spatial delay distribution of said array isconfigured so as to sum the contributions of the incident acousticwave(s) at different spatial positions in the device according to thespatial delay distribution of the device.
 30. The device of anypreceding claim, comprising a first set of unit cells configured toperform a first acoustic manipulation on an incident acoustic wave at afirst operating wavelength and a second set of unit cells configured toperform the same acoustic manipulation on an incident acoustic wave at asecond operating wavelength and further comprising a masking element forselectively blocking either said first set of unit cells or second setof unit cells to switch the operating wavelength of the device.
 31. Thedevice of any preceding claim, comprising a first set of unit cellsconfigured to perform a first acoustic manipulation on an incidentacoustic wave at a fixed operating wavelength and a second set of unitcells configured to perform a second acoustic manipulation on anincident acoustic wave at the same operating wavelength and furthercomprising a masking element for selectively blocking either said firstset of unit cells or second set of unit cells to switch the operation ofthe device.
 32. The device of any preceding claim, comprising a stack oftwo or more layers of unit cells, each layer comprising an array of unitcells.
 33. The device of claim 32, wherein each of said two or morelayers of unit cells has a different spatial delay distribution and/or adifferent spatial configuration of unit cells so that each layer of unitcells performs a different manipulation on an incident acoustic wave,and wherein said two or more layers of unit cells are stacked so thatthe different manipulations are added together as the incident acousticwave passes through the two or more layers of unit cell.
 34. The deviceof any preceding claim, further comprising an acoustic waveguide forguiding said incident acoustic wave onto or towards said array of unitcells.
 35. A structure comprising a device as claimed in any precedingclaim.
 36. A method of generating a spatially modulated acoustic fieldcomprising: generating an acoustic input wave; and passing said acousticinput wave through a device as claimed in any of claims 1 to 8 or 11 to34, or a structure comprising such a device as claimed in claim 35, soas to manipulate said acoustic input wave to generate a desired acousticoutput.
 37. A method of acoustic imaging or sensing comprising: passingan acoustic input wave through a device as claimed in any of claims 1 to8 or 11 to 34, or a structure comprising such a device as claimed inclaim 35, so that the device manipulates the acoustic input wave togenerate an acoustic output; and detecting the acoustic output at asensor or detector.
 38. The method of claim 36 or 37, further comprisingre-configuring said device to generate a different acoustic output. 39.A method of designing or configuring a device for manipulating acousticwaves comprising a plurality of unit cells arranged into one or morelayers each layer comprising an array of unit cells, at least some ofsaid unit cells being configured to introduce time delays to an incidentacoustic wave at the respective positions of the unit cells within thearray(s) of unit cells, such that said plurality of unit cells define anarray of time delays to thereby define a spatial delay distribution formanipulating an incident acoustic wave to generate an acoustic output,the method comprising: determining a quantised delay distribution of adesired analogue acoustic field containing a set of discrete pairs oftime delay values and spatial positions representing the distribution oftime delays required in the device for generating the desired analogueacoustic field; mapping the quantised delay distribution of said desiredanalogue acoustic field to the positions and time delay values of theunit cells within the device; and selecting the time delays for the unitcells within the device based on said mapping.
 40. The method of claim39, comprising a step of compressing the quantised delay distribution inthe spatial and/or delay domains in order to generate a compressed delaydistribution, wherein the resolution of the compressed delaydistribution in the spatial and/or delay domains is such that thecompressed delay distribution may be mapped directly to the positionsand time delays of the unit cells for the device.
 41. The method ofclaim 40, wherein said step of compressing the quantised delaydistribution in the spatial and/or delay domains in order to generate acompressed delay distribution comprises decomposing the quantised delaydistribution into a sum of two or more terms such that each of the twoor more terms of the decomposed delay distribution may be mappedrespectively to two or more layers of unit cells of the device.
 42. Themethod of any of claims 39 to 41, comprising determining a non-uniformdistribution of phase or time delay values required to reproduce thedesired analogue field with a desired accuracy, optionally wherein saiddetermination is based on said step of compressing the quantised delaydistribution.
 43. The method of any of claims 39 to 42, comprisingdetermining the minimum subset of unit cells and/or phase or time delayvalues from an available set of unit cells and/or phase or time delayvalues that is to reproduce the desired analogue field with a desiredaccuracy, optionally wherein said determination is based on said step ofcompressing the quantised delay distribution.
 44. The method of any ofclaims 40 to 43, wherein said compressing step uses a generic featurepreserving compression algorithm such as a wavelet transformation. 45.The method of claim 44, wherein said compression algorithm is a Haarwavelet transformation.
 46. The method of any of claims 39 to 45,further comprising a step of providing as input an accuracy at which itis desired to reproduce the desired analogue acoustic field.
 47. Themethod of any of claims 39 to 46, further comprising manufacturing aplurality of pre-configured unit cells to match the selected timedelays.
 48. The method of any of claims 39 to 46, further comprisinggenerating a control signal for configuring a plurality ofre-configurable unit cells to match the selected time delays.